Personally I have a mirrored drive set that I use as my primary storage, and a third disk that I rsync to nightly and use for backups of other stuff, because I don't trust that I won't have filesystem errors or controller problems that might render both disks in the mirror unusable. Then on top of that I use Crashplan to ensure I have an offsite copy.

For most of my code I also push to github or private external repositories regularly and/or pull to my laptop.

Maybe a bit paranoid, but I've had to recover from enough scary hardware and software failures over the years to not want to take risks.