I think having one back up in the house and then another monthly backup hardrive in security bank deposit is pretty good backup.
Last question: Time Machine. I'm mainly doing it to be honest, just for that very rare occurrence when and if, the primary external fails, I'll have the work from that day somewhere. I dont' really need tons and tons of back ups (although that's a nice luxury) because if the overnight backup via Super Duper works -- I'm covered.
The SuperDuper backup is important, but it won't protect you against a file being corrupted. SuperDuper will simply back up the corrupted file. Same thing for a file that gets mysteriously deleted. Once it's gone from the primary volume, the next SD backup will delete it from the secondary volume as well.
Mirroring is useful only as a defense against physical drive failure. It offers absolutely no protection against corrupted files, mysteriously deleted files, or worse, a corrupted disk catalog. At least with SuperDuper you have a few hours to notice the problem and fix it; with mirroring, damage propagates instantly to both drives.
Time Machine protects against all those things. Its only drawback is that its backups are not bootable. (And, it's only one
kind of backup. You really should have two
kinds of backup.)
For offsite backup, I rotate between two Time Machine backups. I actually have three TM backups: one stays at home and does hourly backups. The other two rotate, one in-house and the other off-site at any one time. For the sake of clarity, call them A, B1, and B2.
Once a week, I tell TM to back up to B1 (assuming that's the drive that happens to be onsite) and let it do one backup. Then I tell it to go back to using A for backups. I carry B1 offsite and swap it for B2. As soon as I get B2 home, I again tell TM to do one backup to B2, then switch it back to A. Next week, I reverse the roles of B1 and B2.
In effect, both B1 and B2 contain only weekly backups, each containing a backup that is at worst a little over a week old. Meanwhile, A continues to give me rapid access to recent versions, with the normal mix of hourly, daily, and weekly backups. A SuperDuper backup completes the coverage.
That sounds like a lot of backup, but I've had TM backup drives fail on me, and it's disconcerting to have all that history disappear. I haven't ever lost any data, but just like in a car, any time you use your spare you no longer have a spare. Besides, that history is useful for other things besides recovery. Sometimes you just want to know what changed and when.
(Case in point: it once was important to me to know when a particular preference got set. The modify date on the preference file wouldn't tell me, because that file got updated several times a day. But TM still had many versions of that file, and it was easy to find the first one that had the particular preference set. Losing your only TM backup makes such questions suddenly unanswerable.)
If recovery requires that I actually use my offsite backup, I've probably also lost the computer itself to fire or burglary, and the extra hour or so it takes to recover from a non-bootable backup pales beside the time it takes to replace the hardware.
