In picking an external disk to use with Time Machine or cloning (or even for use on a network), the temptation is to pick a cheap, simple hard disk, and though that’s fine, you can give yourself some extra protection by choosing a RAID disk.
RAID disks use two or more hard disks inside a single enclosure, and while they can be configured in increasingly complex ways the more disks they have inside them, for our purposes the key thing is that one of these ways is to mirror the contents of one of the internal drives to the other constantly, automatically.
When new data arrives, it’s written to both disks at the same time. This provides extra redundancy whether you’re backing up to it using Time Machine or a cloning app such as SuperDuper, so that even if your internal drive fails and even if one of the disks inside the RAID fails at the same time, you still have one good copy of your data. Backup is all about mitigating risk, and this is a classic way to do it.
The two disks just appear as a single disk as far as your Mac is concerned, so there’s no added complexity.
A quick aside to encourage you to check out the enclosures from Drobo; they don’t use traditional RAID systems, but they give the same redundancy benefits, and allow you to mix and match drives and to grow your storage cheaply and organically in a really useful way. Pricey, though.
Good because: Extra redundancy, with no extra complexity.
But be aware that: Obviously, they’re more expensive. No protection against local disasters.
Fire- and waterproof disks
Companies such as ioSafe make disks that can withstand fire and flooding, and while these still don’t offer you any specific protection against someone breaking into your home and office and stealing your stuff, they at least give some peace of mind by guarding against local catastrophes—at least for a while. Be sure to check the ratings for what they can withstand.
You can get fire- and waterproof disks that connect to your Mac like any other regular hard disk, and, as above, a NAS—which also works with Time Machine.
Good because: Some protection against local disasters, and can be used for Time Machine (direct or over a network) or cloning.
But be aware that: They tend to be bulky and cost a little more. No specific protection against theft.