Plan a Backup Strategy - Take Control of Backing Up Your Mac - page 10

Backing Up iTunes Store Purchases
Apple lets you re-download purchases from the iTunes Store later
(see this Apple support page for details), and if you subscribe to
iTunes Match, you can get an online copy of all your music—even
tracks not purchased from Apple (see iTunes Match and Music
Backups ) . So, to save space on your backup media or time uploading
to cloud services, you might reasonably choose to omit such files
from your backups. However, note that purchased media such as
TV shows and movies could become unavailable later, if Apple stops
selling them in the iTunes Store. And, downloading lots of large audio
and video files can be quite time-consuming.

If you decide to back up your iTunes purchases separately:

Be sure to include not only the files in your iTunes folder (usually
located in ~/Music ) but also the /Users/Shared folder in your
versioned backups; this folder contains hidden information
required to enable authorization.

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If your Mac suffers a severe crash and you decide to erase your
hard disk (to restore all your data from a backup), deauthorize
your computer first. (This prevents you from losing one of your
five authorizations if your computer turns out to require a major
repair.) To do this, open iTunes and choose Store > Deauthorize
This Computer. Then, in the dialog that appears, authenticate, and
click Deauthorize. After restoring your backup, reauthorize the
computer in iTunes by choosing Store > Authorize This Computer.

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Why Use an External Hard Drive?

Not so long ago, using a hard drive for backups was considered way too
expensive compared to, say, a stack of CD-R discs. Today, hard drives
offer the highest capacity of any storage medium plus extremely fast
performance and low cost. You can also make a backup onto a hard
drive in such a way that you can start up your Mac directly from the
backup—a trick you can't do with most other media. For reasons of
capacity, speed, cost, and convenience, external hard drives are ideal.

I want to emphasize the word external . Some Mac models can
accommodate more than one internal hard drive. And on any Mac,