Mac OS X – Adding an External Disk Drive

We often find ourselves in the position of needing another disk for our system. External drives can be used for added data storage, as a simple backup device, as a Time machine drive, or as a complex
multi-disk RAID array for video production [1]. Adding an external drive to your Mac is very quick and simple. However, there are several questions that you need to ask your self before buying an external disk. That is what this article is about.

How Are You Going to Connect the External Drive?

There are different connection methods that you may have available
on your Mac for adding an external drive. Depending on when your Mac was built and which model it is, your Mac may have a Firewire, a USB and/or a Thunderbolt port for connecting external devices. Actually there are more types of connections than just these three, as there are sub-flavors of both USB and Firewire.

To see what your Mac has available, click on the Apple in the top left corner of your screen >> About This Mac >> More Info >> System Report. Under Hardware look for the Firewire, USB and Thunderbolt entries. You can see which interfaces your Mac has available and which speed they support. When I checked my Mid 2011 Mac Mini, I found that I have Firewire 800, USB 2.0 and Thunderbolt.

Connection Method Speed (megabits/s) Order of performance
Firewire 400 (IEEE 1394-1995) 393.216 4th
Firewire 800 (IEEE 1394b-2002) 786.432 3rd
USB 1.1 12 6th
USB 2.0 480 (effective throughput is limited to 280) 5th
USB 3.0 5000 (usable up to 4000) 2nd
Thunderbolt 10000 (actually 20000 as Thunderbolt supports
two channels at 10000 each)


Obviously the faster connection you use the better, but there may be extra costs. USB 3.0 may cost more than USB 2.0, and Thunderbolt will definitely be the most expensive. Since USB 3.0 devices are downwards compatible (they will still work in a USB 2.0, or even a USB 1.1, port, though at down graded performance) you can buy a drive with a faster connection interface than your Mac currently has and it will still work. Since USB 3.0 has become the most common port, most manufacturers have dropped producing USB 2.0 only compatible drives.

Many drive manufacturers build their drives with multiple interfaces
– combination Firewire 800 / USB 3.0 drives are very common.

Most Likely Answer – The general case recommendation if you are buying an external drive today is a Firewire 800 / USB 3.0 Combo (or just USB 3.0) [2]. Generally you would only go to the expense of a Thunderbolt drive if you were primarily running an application that requires a lot of high speed file I/O, such as video editing.

What type of Drive Should I Choose?

There are choices to be made as to the type of disk as well.

SSA – A Solid State Drive will give the most performance, but it will have the highest cost per gigabyte of available storage. They are
highly reliable since they have no moving parts [3] and are extremely quiet for the same reason. A SDD makes much more sense as a boot drive than as an external drive in most cases.

HDDHard Disk Drives are the most common choice, but even then there are alternatives 5400rpm and 7200rpm rotating speeds (Yes. HDDs also come in 10000rpm and 15000rpm, but those are not likely to be available as external drive choices).

Disk rotation speed affects disk performance. For instance, a 5400rpm disk will have a 5.55ms average latency, while a 7200rpm disk will have only a 4.16ms latency. If the applications you generally use perform a large amount of disk I/O, the rotational speed will make a difference. If not, then you will generally be better to go with a 5400rpm drive. For a given price point, you can get a larger capacity drive at 5400rpm.

Most Likely Answer – The general case recommendation would be 5400rpm, unless the application you are using involves a lot of high speed disk I/O. If your budget allows, go for the 7200rpm drive.

Should I get a desktop hard drive or a portable hard drive?

Desktop Drives – The simplest way to differentiate an desktop
versus a portable drive is that a desktop hard drive requires an
external power source, while a portable hard drive can be powered
just by your computer [4]. The desktop’s power supply may seem
unnecessary, but many of the desktop drives will include a fan to keep the drive cooler under heavy operation [4]. In general, the
desktop drives will be able to perform under sustained heavy use
better. If I wanted to move my iPhoto library to an external drive
for my Mac Mini and keep it constantly attached and powered on, I
would choose a desktop drive.

Portable drives – They provide the benefit identified by their name, they are easy to take with you. Since they only need to connect to your computer by the USB, Firewire, or Thunderbolt connection, they are easy to set up, and easy to drop into your computer bag and take along with your laptop. Many portable drives are ruggedized to take the heavier punishment that constantly moving it around can exert. I use a portable 1 TB (terabyte) drive for my Time machine backups. Once a week I attach the drive, update my backup, then put the drive back in a desk drawer.

Most Likely Answer – This depends. Choose a portable drive if portability is needed, or if you are only going to occasionally attach the drive to your computer. If you are leaving the disk attached and running all the time, go for the desktop drive.

How Large of a Drive Should I Get?

When you are choosing an external drive, you should buy enough
space to accommodate your needs for the foreseeable future. How
much space is enough? That depends on what kind of files you are going to be storing, but in any case always get a bigger drive than you currently think you will need.

On the other hand, choosing the largest drive available may not be a
good idea as the largest disks are often at a premium price. Compare
the available drives and see what the price per GB (gigabyte) is (disk cost / disk capacity). Usually you will see that the cost per GB goes down as the size of the disk increases.

Most Likely Answer – buy the largest drive that you can afford. Even if you cannot imagine needing the space now, more than likely you will use it eventually.

Should I Consider Building Instead of Buying the Drive?

Yes, you CAN build your own external drive! Certainly the pre-installed external drives are more popular, but building your own external drive is worth considering.

Pre-installed – The pre-installed external disk is the most popular. Pre-installed external drives come completely assembled with the drive size you specify. They include a warranty that covers the case, drive, cables, and power supply. All you need to do is plug the external drive into your Mac, format the drive, and you’re ready to go.

If you do not have an extra, unused disk drive of the needed capacity, buying a drive enclosure and a new disk drive will probably be more expensive than just buying the pre-installed unit.

Assemble Yourself – Installing a hard drive in an external disk enclosure is an easy task that almost anyone can perform [2]. If you want to assemble your own external disk, then you would buy a hard drive enclosure. It would come with a power supply and hard drive interface. All you would need to do is install the disk drive. The drive you use does not have to be new. Working drives from old computers are often re-purposed as external drives.

External disk enclosures come in three sizes: 1.8″, 2.5″ and 3.5″. This refers to the physical size of the hard disk it can contain. You would pick the enclosure size to match the available drive. Usually you would choose a 3.5″ enclosures unless you are simply trying to make use of an old 2.5″ laptop drive. Power to the enclosure may be supplied by the connecting cable (USB, Firewire, Thunderbolt), but some use an external power supply (wall wart). The 3.5″ enclosures are more likely to have external power adapters.

Also check on the interface internal to the enclosure for the disk itself. The internal interface connecting the drive to the enclosure is usually a SATA (Serial ATA) 2 (3 GB per second) or SATA 3 (6 GB per second) interface [1]. If the enclosure has a USB 3.0, Firewire 800 or Thunderbolt connection to the computer, you will want to insure that the internal interface is the faster SATA 3 and that the drive you are using is compatible with that interface.

Choosing to install the drive yourself usually offers more options. There are many different case styles, and choice in the type of supported connections. You also get to choose the size and brand of the disk. If you are buying a new drive, then you may be able to get a longer warranty on it than you would a pre-installed drive [1].

If you need very large capacity, high I/O or redundancy, then you
may want to pick a muli-bay enclosure. These allow for more than
one disk to be installed, and usually configured in a RAID array.

Most Likely Answer – Both options are good choices. It is mostly a matter of your needs, your skills and your interested level.

Do I Need a RAID Array?

So, what is RAID? If you have the right enclosure with two or more disks, then you can configure the disks into a RAID (Redundant Array of Independent Disks) array. Depending on the number of drives you can configure them as:

RAID 0 – two identical drives where I/O is stripped across the pair (part of the data is written to each disk). This gives no redundancy, but it does improve performance and the capacity of the array is the sum of the two drives. Failure of one drive renders the array unusable.

RAID 1 – two identical drives where the data is mirrored (written identically to both). If you loose one drive, the array continues to function with no data loss. The capacity of the array is the capacity of one drive.

RAID 5 – requires at least three identical disks. Data is written across all disks with distributed parity. If one disk fails, it just needs to be replaced. Because of the distributed parity, the data on the failed disk will be rebuilt on the new drive. No data will be lost from a single disk failure. Capacity of a three disk array is the size of two disks (in a RAID 5 array of N disks, the array capacity will be the total of N-1 disks).

Most Likely Answer – No for most users. RAID is mostly used for those instances where you need a larger capacity than a single disk will provide OR faster I/O than a single disk is capable of OR you need the reliability RAID provides.

Should I Only Consider Local External Drives?

Adding external drive space does not necessarily mean that you connect a disk drive directly to your computer. In some situations using a remote disk makes more sense [5].

Local drives – Local drives are those connected to the computer over USB, Firewire or Thunderbolt cables. Local drives usually have a speed advantage over those connected over a network [5]. Local drives also have the benefit of little or no setup required. In many cases you just take it out of the box, connect it to the computer and you are ready to use it. You may also find that some applications will only work with local drives. Using a local external drive also means that you can take the drive with you to a new work location if needed.

Network Attached drives – You should consider a Network

Attached Storage (NAS) device if there are multiple computers in your home or office that need an external disk. A NAS is essentially a special purpose computer with one or more disks attached. It’s only purpose is to store data (act as a file server).

The disks may be configured in a RAID.

Network attached drives either connect via an Ethernet cable to your home/office network, or in some cases connect with client computers over Wi-Fi. While Wi-Fi connections are very convenient, they are also generally slower. Because of the network connection, setting up a network attached drive and then configuring all of the computers to use it generally requires more in-depth knowledge. Since there are in essence a small computer, network attached drives will be more expensive.

Your OS X Mac can work with many NAS devices. OS X supports Windows file sharing protocols, such as Common Internet File System (CIFS), as well as Apple’s own Apple Filing Protocol (AFP).

Be aware though that Time machine will only work with a network attached drive which supports AFP [5].

Apple does have network attached storage products. Apple offers
the AirPort Extreme ($99) that allows an external USB disk to be connected and shared. Apple also offers the AirPort Time Capsule (2TB $299, 3TB $399) which has an internal drive that can be shared.

Most Likely Answer – Yes, for most users a local drive is the answer.

Should I use a Cloud Service?

There are many cloud storage services available. It is an obvious questions if using those is a viable option for external storage.

Cloud storage – Access to cloud storage is going to be significantly slower than the other possible solutions. There is also the concern as to how secure the data is that is being stored on the cloud service. If the solution does not allow user side encryption, then content stored on cloud sites may be vulnerable, and would certainly be available to law enforcement with a warrant.

There are many cloud storage services, some even offer a limited amount of free disk space. The cost of cloud storage is, at this point, much higher than purchasing a local external drive. This is especially true when you consider a three or five year cost comparison.

Cloud storage solutions do provide you with off site backup if that is desired. They will allow access from anywhere there is network access and many offer some sort of shared access for multiple users.

Using a cloud storage service is a good way of sharing work between computers. I use Dropbox to have common access to files I am working on (like these articles) between my MacBook and Mac Mini. I can then work in my office on my Mac Mini, or go sit in the living room and watch TV while working on the same file. However, I move the files once they are complete to the disk drive on my Mac Mini for archival storage.

You may want to take a look at this very thorough comparison of cloud storage services. Some of the more popular are (data captured 9/8/14):

  • Dropbox – 2 GB free, $9.99/month 1 TB
  • Box – 10 GB free, 100 GB $5/month, unlimited $15/month
  • Google Drive – 15 GB free
  • iCloud – 5 GB free
  • Spideroak – 2 GB free, 100 GB $10 / month
  • Wuala – 5 GB $1.39/ month, 20 GB $3.99/month, 50 GB $6.99/month

Non-cloud storage – By non-cloud storage I refer to either a locally attached disk or NAS device located in the same home or office as the computer. These will always be faster, and will most likely always be lest costly.

Most Likely Answer – No, unless you need off site backup or the ability to share files with others.

Can I just use a Flash Drive?

We all have a few USB Flash Drives that we have picked up over the years. It is reasonable to think about using those for external storage. While these work fine for moving content between computers, they are not good choices for true external storage. The I/O rates, particularly when writing data to the drive, are far lower than for the other storage media I have discussed. The cost per GB is
also going to be higher. These drives also can wear out as they have a low limit on the number of times data can be written to the drive, with some only lasting for 3000 to 5000 cycles of writing
new data.

Saving the only copy of important files on a USB Flash Drive is not
a good idea. Because of their size they are easy to misplace unless
stored in a secure location, and I have sent at least one USB Flash
Drive on a wild ride through my washer and dryer (amazingly that one still worked afterwards, though I have no idea where it is now).

USB Flash Drives do make sense if you have apps or files you want
to carry with you to use on different Macs.

Most Likely Answer – No, generally, though they do have a place for special purpose use.

Is There Anything Else I Should be Aware of?

There are several disk drive specifications that you can look at, particularly as you compare one external drive to another:

Seek time – how long it takes for the heads to travel to the track with the data (a lower number is better)

Latency – delay for the rotation of the disk to bring the desired data under the heads (a lower number is better)

Data Transfer Rate – how long to transfer data from the disk to the buffer. Depends upon rotational speed and data recording density (higher number is better)

Disk Buffer – memory in the disk drive unit that holds recently read sectors, and nearby sectors that may be requested in the future. If the desired sector of data in in the buffer, the data can be returned much more quickly than if it has to be read from the disk itself. (Larger buffers are usually better)

What Kind of Budget Should I Expect?

So how much should you expect to spend? I spent a short time pulling some cost data, but of course this is subject to change on a moments notice. Nor did I do an exhaustive search to find the best prices.

You would want to check with your favorite computer store and see what the current prices are. Most likely you will get the best deals from an on-line store if you are willing to wait a few days for delivery.

The three items listed with 0 GB of storage are empty enclosures that you can add your own drive to.

The cost per GB of storage for those drives supporting RAID is calculated on the usable storage available, not the total storage.

Type Drive Capacity (GB) Vendor Price Cost per GB
USB Flash Drive SanDisk 32 Amazon $12.99 $0.41
USB Flash Drive PNY 128 Amazon $39.95 $0.31
USB Flash Drive Patriot 256 Amazon $129.98 $0.51
Portable USB 3.0/Firewire 800 OWC Mercury 0 OWC $59.99
Portable USB 3.0 OWC Mercury 0 OWC $29.99
Portable USB 3.0/Firewire 800 OWC Mercury 7200 rpm 500 OWC $125.99 $0.25
Portable USB 3.0 OWC Mercury 7200 rpm 500 OWC $97.99 $0.20
Portable USB 3.0 OWC Mercury 5400 rpm 500 OWC $79.99 $0.16
Portable USB 3.0/Firewire 800 OWC Mercury 7200 rpm 1000 OWC $149.99 $0.15
Portable USB 3.0/Firewire 800 OWC Mercury 5400 rpm 500 OWC $109.99 $0.22
Portable USB 3.0/Firewire 800 OWC Mercury 5400 rpm 1000 OWC $139.99 $0.14
Portable USB 3.0/Firewire 800 OWC Mercury SSD 480 OWC $349.99 $0.73
Portable USB 3.0 OWC Mercury SSD 480 OWC $319.99 $0.67
Portable Thunderbolt OWC Mercury 1000 OWC 169.00 $0.17
Desktop USB 2.0 / Firewire 800 OWC miniStack 0 OWC $39.00
Desktop USB 2.0 / Firewire 800 OWC miniStack 7200 rpm 500 OWC $97.99 $0.20
Desktop USB 2.0 / Firewire 800 OWC miniStack 7200 rpm 3000 OWC $169.99 $0.06
Desktop RAID 5 USB 3.0 / Firewire 800 OWC Elite Pro 7200 rpm 4000 (3000 for data) OWC $524.00 $0.18
Desktop RAID 5 USB 3.0 / Firewire 800 OWC Elite Pro 7200 rpm 12000 (9000 for data) OWC $799.00 $0.09
Desktop RAID 5 USB 3.0 / Firewire 800 OWC Elite Pro 7200 rpm 24000 (18000 for data) OWC $3149.00 $0.18
NAS RAID 51 Drobo 5N 20000 (16000 for data) Amazon $1499.00 $0.09
NAS LaCie 2big NAS 4000/RAID 0 (2000/RAID 1) Amazon $349.00 $0.09

1 The Drobo does not use true RAID 5, but something

What do I need to do to Use the Drive?

Your Mac uses the HFS+ filesystem while Windows computers use
NTFS. Linux based computers use yet other filesystems. The external disk you purchase will only be formatted one way, often with NTFS as the majority of computers are currently Windows based.  As mentioned above, your Mac with OS X will be able to access an NTFS formatted disk. If you are wanting to share the disk between multiple computers running different operating systems, then NTFS is going to be the best choice. OS X, Windows and Linux systems can all access a NTFS formatted drive.

Most Likely Answer – For Mac exclusive use, reformat the drive to HFS+.

To format the drive attach the drive to your Mac >> open a Finder window >> open Applications >> open Utilities >> double click on Disk Utility. Select the drive device to be formatted (above list of storage volumes on the device, often with the manufacturer name and size  CAUTION – be sure you are selecting the correct drive to format! ) >> choose ‘Partition’ tab >> select ‘1 Partition’ from drop down menu >> click the Options button and ensure that ‘GUID’ is selected as the partition scheme >> click the Apply button [6].

In the example shown below I am reformatting the 160.04 GB Seagate Freedom device. Format



[1] – Increase Storage With an External Drive for Your Mac
[2] – Before You Buy an External Hard Drive
[3] – The best hard drives and flash storage for your Mac
[4] – 3 Tips for Buying a Portable or External Hard Drive
[5] – Room to Grow: Data Storage for Your Mac Explained
[6] – How to set up an external hard drive for use with OS X

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