why not use RAID-1? that's exactly the kind of thing it's for. in fact, raid-1 IS "mirroring".
NOTE: RAID is NOT a substitute for backups. you'll still need to backup your data regularly, to tape or something....but a RAID array will both reduce the likelihood you'll need to run a full restore (always a PITA), and keep the system up and running on the single remaining disk while you swap the dead drive ASAP.
btw, for added redundancy you can add a third (or fourth etc) drive to the RAID-1. you'll still have the same storage capacity as just one drive, but all 3 (or 4 etc) drives would have to die simultaneously for your system to be down.
or, with 3+ drives you could use RAID-5 - but ONLY if you have a good hardware raid card with non-volatile cache. this emphatically does not include the fake-raid rubbish commonly found on motherboards - that's really just software raid with a special driver.
with R5 you get (n-1) storage capacity where n=the number of disks in the array....but write speed is abysmal unless you have a good cache. and it needs to be non-volatile to avoid corruption in case of power failure or crash.
the catch with raid-5 is that if you have 3 drives in the array, if one disk dies you're OK...but if two disks die, you lose everything. again, you can increase the redundancy by adding hot-spares, similar to adding extra disks to RAID-1.
or if you have 4 drives or more, you could use RAID-6. glossing over the details, it's pretty similar to raid-5 except that you have (n-2) capacity, and you can afford to lose two drives in the array before you lose the lot.
personally, i like and use RAID-1 the most. it saves your data without imposing a huge performance penalty. i'll use it in preference to R5/R6 unless disk space is at least as important as redundancy.