Information Technology Reference
Your Mac refuses to start up—perhaps with a blinking question
mark icon, or with a blue or gray screen that never goes away.
You begin noticing misbehavior in multiple applications, such
as failure to launch, incorrect preferences, or missing documents.
In situations like these, you're looking at some down time. Maybe
your computer is out of commission for a half hour while you quickly
run a disk repair utility; maybe it's out for days while you wait for
a replacement hard drive to be delivered. In any case, there's going
to be a period of time during which you can't get any work done. For
many of us, myself included, that's a serious problem. Even though
all modern Macs can boot from a hidden Recovery HD volume or, in a
pinch, over the Internet thanks to OS X Internet Recovery (see Apple's
OS X Internet Recovery page for details), booting this way doesn't give
you immediate access to your applications and data.
That's why, in addition to versioned backups, I recommend creating
a bootable duplicate. You'll store a complete copy of your startup disk
on another drive, such that if your main disk ever goes south, you can
start up your Mac—or even a different Mac—from your backup drive
and get back to work in minutes (instead of hours or days). Bootable
duplicates also give you insurance against software updates gone bad.
If you install a new version of Mac OS X and encounter compatibility
problems, you can quickly revert your disk to the way it was before.
The only real decisions you have to make concerning duplicates are
which software to use and how often to update your duplicates. I
discuss that in Create a Bootable Duplicate .
Tip: Another good reason for bootable duplicates is that they make
hard drive upgrades painless. Simply follow your existing procedure
to make bootable duplicates, this time choosing your duplicate as
the source and your new drive (say, a larger model or an SSD) as
I told you that a handful of people may not need versioned backups; is
the same true of bootable duplicates? If you already have another way
to boot your Mac in an emergency, if you have versioned backups of all