Information Technology Reference
which can't store bootable duplicates, you'll use an entirely separate
drive for this purpose.) But the volume that stores your duplicate need
only be as large as the amount of data on your startup volume, not
necessarily the whole disk. For example, if your Mac came with a 1 TB
hard drive but you've filled up only 500 GB of that space, you can fit a
duplicate on a 500 GB disk or partition.
Over time, though, you'll add more files to your Mac, so if you cut
it that close, you'll soon outgrow your backup drive. Therefore, I
suggest you allot at least one and a half times the amount of space
currently occupied on your startup volume for a duplicate. So if
you have 500 GB of data on your startup volume, you want (at least)
750 GB for the duplicate. More space, of course, is perfectly fine; it
will give you even more room to grow.
To find out how much space on your startup volume is being used,
select the your disk's icon in the sidebar of any Finder window. Then
press Command-I to display the Info window ( Figure 1 ). The number
by “Used” is the amount of space currently occupied on the disk.
Figure 1: To see how much space is occupied on your disk, select it,
choose File > Get Info, and look for the number next to “Used.”