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continuously, and update my bootable duplicates twice a day. But
that's me.) In no circumstances do I suggest backing up less often
than twice a month—the risk is too high.
Tip: Regardless of your schedule, always update your duplicate
manually just before installing system software updates. That way,
if the new version has serious problems, you can easily roll back
your Mac to its previous state.
If you're actively working on an important and time-sensitive
document, even hourly backups, such as those offered by Time
Machine, may not be enough. You may want to supplement your
ordinary versioned backups with software that stores every single
version you save. Many apps running under Lion and later can do
that automatically, and other options exist if that approach won't
work for you (refer to Version Control for details).
Why Keep Multiple Backups?
A sound backup strategy always includes backups of your backups!
Picture this: You've diligently backed up your computer's internal hard
disk to an external drive. One day, lightning strikes and both drives are
damaged. So much for your backup! Even under ordinary conditions,
backup media can fail for all the same reasons your hard drive can fail.
So having just one backup, in my opinion, is never enough. You should
alternate between two or more sets of backup media for greater safety.
If you've set up your backups to run on a schedule, this might mean
using media A every day for a week, then switching to media B for each
day of the following week, then switching back—and so on.
So are two sets enough? It depends. To protect against media failure,
most experts recommend using at least three sets, of which one is
always stored offsite. But using online backups (see Use an Internet
Backup Service ) counts as at least one set, and perhaps more if your
online backup provider keeps its own internal backups. If you're not
backing up online, using three hard drives does make rotating media
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