Choosing a Time Machine Backup Drive

There are several issues to consider in choosing a drive for Time Machine:

• Capacity for current and future needs;
• Performance: a slow drive can interrupt your work or take a very long time to finish. A USB drive is therefore a poor choice;
• Convenience, location, etc: internal or external, at your Mac or in a closet, etc.

These issues are discussed below.

Capacity of the Time Machine backup drive

Time Machine keeps a backup of everything that you don’t explicitly exclude (except for some types of external drives).

In addition, Time Machine keeps versions of files as they change. Lacking enough space, Time Machine will forget about older versions to make space for newer versions, so be sure to provide a large enough backup volume to get maximal benefit of the versioning feature.

For a Time Machine volume, use a drive of at least 50% greater capacity than all your data so that Time Machine can keep its history (so you can “go back in time”).

A figure of 30% should be considered an absolute minimum, double the space of your data is ideal eg a 2TB backup drive for 1TB of data.

Note that this guideline is about how much data you actually have. For example, you might have a 2TB drive with only 200GB of data. You therefore could quite reasonably use a 500GB backup drive for Time Machine, at least until such a time as your data grows much larger.

TIP — If your Time Machine volume cannot hold everything, exclude older items on the backed-up volumes— so much material is static (unchanging) and thus should be backed up to at least two or three external backups. Time Machine is best used for protecting yourself for recent and changing files.

It is best to have formal backups (a minimum of two), and thus Time Machine should be thought of as a short-term protection system. While it can be used for external backups, cloning multiple copies is a far better approach for formal backups in conjunction with Time Machine (which has its own dedicated backup drive which is always attached or internal).

OWC Thunderbolt 3 Dock
Ideal for any Mac with Thunderbolt 3


Dual Thunderbolt 3 ports
USB 3 • USB-C
Gigabit Ethernet
5K and 4K display support plus Mini Display Port
Analog sound in/out and Optical sound out
Works on any Mac with Thunderbolt 3

MPG discussion of OWC Thunderbolt 3 Dock

Which hard drive?

A high capacity unit with USB3 is the best option as of 2014. Many users will find 3TB capacity enough, but go with 4TB to extend the years of use without filling up.

Older Macs without USB3 should use eSATA if possible, then Firewire 800 if eSATA is not an option.

Apple’s Time Capsule

Apple’s Time Capsule is an option for some, but its price is relatively high. However, the Time Capsule can also backup another Mac, which brings its relative price down.

Time Capsule has the appeal of being available on a network, so you can locate it away from your Mac. However a wireless network might not deliver acceptable performance (interrupts/degrades use of the computer).