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internal drive plus an external drive of the same capacity is effectively
the same thing as a duplicate—only better, because it's always
100 percent up to date. I beg to differ.
I have nothing against mirrored RAIDs. However, a RAID, by itself,
is no substitute for multiple duplicates as described in this topic. A
mirrored RAID's best feature is also its Achilles' heel: because changes
are reflected on all drives at once, an accidentally deleted (or damaged)
file will be immediately deleted (or damaged) on your “backup” drives
too! (Stand-alone duplicates—especially if you maintain two or more
of them—reduce this risk greatly.) RAIDs address the problem of drive
failures, but they provide no insurance against human error, theft, or
any of the other catastrophes that make backups so important.
That said, you can have your cake and eat it too (for a price). If you
use SoftRAID ($129), you can create a RAID in which your internal
disk is mirrored onto two or more external drives at once. You can
then periodically rotate one of the drives offsite, where it will function
as a stand-alone duplicate of your disk at an earlier state. When you
plug it back into your Mac, it will automatically synchronize with the
remaining drives in the RAID. The beauty of this approach is that
you never have to set up, schedule, or run backup software to make
duplicates—it happens automatically.
This scheme can be expanded to include versioned backups. Using
SoftRAID, it is possible (though a bit awkward) to partition an external
drive so one partition is used along with your internal drive to form
a mirrored RAID while another, non-RAID partition on the external
holds versioned backups. Set up two drives this way and you're as close
to a bulletproof backup system as you're likely to get.
The BeyondRAID system in Drobo devices also provides data
redundancy, and can (in some cases) be used as a bootable duplicate.
However, Drobo's benefits and limitations are fundamentally different
from those of a conventional mirrored RAID, as I explain next.
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