Information Technology Reference
In-Depth Information
Tip: If you can't locate your startup volume in the Devices category
in the sidebar, choose Finder > Preferences and select its checkbox.
Versioned Backup Size
Time Machine requires that your destination volume have, as a bare
minimum, 1.2 times the space occupied by the data you're backing up.
(That gives some extra space to store multiple versions of at least some
files.) So, as a first pass, multiply the “Used” value you saw in the Get
Info window by 1.2 to find out the smallest partition size Time Machine
can use. (If you plan to back up additional disks, be sure to factor in
their sizes before multiplying by 1.2.)
But I must emphasize that 1.2 times is an absolute rock-bottom
minimum. You'll be far better off setting aside 1.5 times, 2 times, or
even 3 times the amount of space used on your disk for backups. The
reason is simple: the more space Time Machine has to work with on
the destination drive, the more backups it can store—and the farther
back in time you can reach when you restore data.
What if you're not using Time Machine? Well, the general principle
still holds that you'll want more free space than is currently occupied
on your drive, with a bit of a cushion. A twist is that most other backup
software offers compression, deduplication, and/or delta encoding, all
of which make your files take up less space on the backup drive than
they otherwise would. So, for most people, having free space equal to
1.5 times the amount of data you want to back up should be adequate.
Total Size
Unless you're using a Time Capsule, in which you'll have a separate
drive just for a duplicate, your duplicate and versioned backups
will ideally live on the same physical disk, so you must now add those
two numbers together. For example, if you have 500 GB worth of data,
you might choose to allot 750 GB for a duplicate and another 750 GB
for versioned backups, bringing the total to a tidy 1.5 TB. That means:
look for a 1.5 TB or larger-capacity drive. Do consider the next-larger
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