Increase Storage With an External Drive for Your Mac

With so many choices available, external drives are an ideal way to gain storage

External drives may be the most common way to increase a Mac's data storage capability, but they can do more than just provide extra space. External drives are versatile, both in how they can be used and in the types and form factors available. Here's a look at the various types of external drives, how they connect to a Mac, and which type might be the best fit for you.

An external hard drive.
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Types of External Enclosures

This category includes a wide variety of enclosures, from small USB flash drives, which can serve as temporary storage or as a permanent home for apps and data that you need to carry with you, to large drive arrays that hold multiple storage devices in one case.

  • USB flash drives: Small, portable, and relatively inexpensive, these house capacities ranging from 2 GB to 2 TB. The downside is their slowness, especially when you're writing data to them.
  • 1.8-inch external enclosures: Designed to hold a single 1.8-inch hard drive or SSD. Power is usually supplied by the interface bus (USB or FireWire), but some enclosures use external power supplies (wall warts). This type of enclosure should perform as well as any other external device that uses the same type of computer interface.
  • 2.5-inch external enclosures: Designed for use with the types of hard drives and SSDs commonly installed in laptop computers. Performance depends mostly on the type of external interface used to connect the enclosure to the Mac. Common interface options include USB 2, USB 3, and eSATA. The enclosures can be bus-powered or have their own power supplies.
  • 3.5-inch external enclosures: Used with the standard hard drives and SSDs found in most desktop computers. In some cases, two SSDs can be installed in this enclosure size. External interfaces include USB 2, USB 3, FireWire, eSATA, and Thunderbolt. This type of enclosure usually has its own power supply.
  • Multi-bay enclosures: This type of enclosure uses multiple bays or docks. Each bay supports a single drive. Multi-bay enclosures range from holding two drives to holding 16 or more drives. They usually hold 3.5-inch drives, but many also support SSDs. The external interfaces available include USB 2, USB 3, FireWire, eSATA (and other SATA types), and Thunderbolt. Each bay may have its own external interface, or the drives may be routed through a RAID controller and presented to the Mac using a single interface.

Types of Interfaces

External drive enclosures have two types of interfaces: internal and external. The internal interface connects the drive to the enclosure and is usually a SATA 2 (3 Gbps) or SATA 3 (6 Gbps). The external interface connects the enclosure to the Mac. Many external enclosures offer multiple external interfaces, so they can connect to almost any computer. Common interfaces, in descending order of performance, are:

Of the interfaces mentioned, only eSATA lacks a built-in interface on Macs. Third-party eSATA cards are available for the Mac Pro and the 17-inch MacBook Pro, using the ExpressCard/34 expansion slot.

USB 3 is overtaking USB 2, once the most common interface; nearly every new external enclosure offers USB 3 as an interface option. That's a good thing because USB 3 offers much faster performance than its predecessor and FireWire. Even better, the cost of USB 3 devices is in the same range as USB 2. If you're considering a new USB-based device, go with an external device that supports USB 3.

USB 2.0 ports in black at left, USB 3.0 ports in blue at right.

When looking for a USB 3-based external enclosure, keep an eye out for one that supports USB Attached SCSI, often abbreviated as UAS or UASP. UAS uses SCSI (Small Computer System Interface) commands, which support SATA command queuing and separation of transfer types into their own data pipes.

UAS doesn't change the speed at which USB 3 runs, but it makes the process much more efficient, allowing more data to be sent to and from an enclosure in any given time frame. OS X Mountain Lion and later include support for UAS external enclosures; taking the time to find enclosures that support UAS is worthwhile, especially for those that will contain an SSD or multiple drives.

If you're looking for optimal performance, then Thunderbolt or eSATA is the way to go. Thunderbolt has an overall performance advantage and can support multiple drives with a single Thunderbolt connection. This makes Thunderbolt an attractive choice for multi-bay enclosures that contain multiple drives.

Akitio Thunder2 Duo Pro
Courtesy of Akitio

Prebuilt or DIY?

You can purchase external cases that are prepopulated with one or more drives, or empty cases that require you to supply and install the drive(s). Each type has advantages and disadvantages.


A prebuilt external comes completely assembled with the drive size you specify. It typically includes a warranty that covers the case, drive, cables, and power supply. All you need to do is plug the external into your Mac and format the drive. Prebuilt externals can cost more than a DIY external case, which is supplied without any drives. If you don't already have a drive on hand, though, the cost of buying an empty case and a new drive can come close to, and in a few instances, exceed the cost of a prebuilt external. Nevertheless, a prebuilt external is ideal if you just want to plug in a drive and go.


DIY, on the other hand, generally provides more options in case styles, the types and numbers of external interfaces, and the sizes and makes of the drives. Depending on the drive manufacturer and the model you choose, the warranty period for the drive might be much longer than for a prebuilt model. In some cases (no pun intended), the warranty for a DIY model can be up to five years, vs. one year or less for some prebuilt models.

Consider Repurposing

The cost of a DIY external drive can be much less than that of a prebuilt if you're repurposing a drive you already own. If you upgrade a drive in your Mac, for example, you can use the old drive in an external DIY case. That's a great use of the older drive and a real cost saver. On the other hand, if you're purchasing both a new DIY case and a new drive, you can easily exceed the cost of a prebuilt—but you might be getting a larger and/or higher performance drive, or a longer warranty.

Uses for an External Drive

The uses for an external drive can range from the mundane but oh-so-important backup or Time Machine drive to high-performance RAID arrays for multimedia production. Other popular uses for external drives include dedicated media libraries and home folders for user accounts. In fact, the last option is a very popular one, especially if you have a smallish SSD as your startup drive. Many Mac users with this configuration quickly outgrow the available space on the SSD. They alleviate the problem by moving their home folders to a second drive—in many cases, an external drive.

So, Which is Best, Prebuilt or DIY?

Neither option is hands-down better than the other. It's a matter of what meets your needs, skills and interest level. If you like the idea of repurposing items that might otherwise be discarded, you like to tinker, and you're up for learning new skills, there's no end to the uses for old drives.

If you need external storage but have no spare drives on hand, or if you're just not a do-it-yourselfer, a prebuilt external may be the best choice for you.


Whether you choose a prebuilt or a DIY external drive, look for multiple external interfaces. At a minimum, the drive should support USB 2 and USB 3. (Some devices have separate USB 2 and USB 3 ports; some devices have USB 3 ports that also support USB 2.) If you need maximum performance, look for a case with a Thunderbolt interface.

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