"Storing a backup alongside other files on a
destination drive
In general, we strongly recommend that you store a backup on its own
volume or partition, on a FireWire or internal drive that supports booting the
Macintosh. Doing so makes it really easy to recover, saving you time – a lot
of time – should a disaster occur.
And not to toss a wet blanket on the optimist but, eventually, a disaster will
occur.
But, sometimes, dedicating a volume to backing up is impractical, and
partitioning is either inconvenient or impossible. In those cases, you should
use a Read/Write “Sparse” Image.
With a Sparse Image, your files are kept in a “virtual” disk, which is actually
stored in a single file. This file can be placed anywhere, including on a
network volume. When mounted (which you do by simply opening the
Sparse Image file), the “virtual” drive looks just like a local disk, and can be
backed up to like any other. You can even Smart Update it!
There are some disadvantages, though:
• Since Sparse Images are not “real” drives, but rather simulated drives
stored in a file, they’re potentially less reliable than real drives. (Note,
though, that Apple uses sparse images for FileVault, so this shouldn’t
be a big concern.)
• Similarly, since Sparse Images are not real drives, they’re inherently a
bit slower.
• Finally, since they’re not physical drives, images of any type cannot be
used to start up your Macintosh. However, the contents retain their
bootable properties and – once restored to a real drive – can act as a
startup drive.
So, with that in mind, using sparse images is quite simple.
Choose your source drive normally. In the destination pop-up, choose Disk
Image... "