Set up 2 FW800 JBOD units as a RAID 1(mirror)
A RAID setup is not a backup. A RAID is a way of mitigating hardware failure, its value is that despite a piece of hardware failing you can keep working with no down time. In your case, the issue you had a (a failed drive) would have been handled just fine by a RAID, but the following would not:


Accidentally deleting a file (the RAID will happily delete it on both of the mirrored drives.
Data corruption due to bugs in any programs you are using (again, the RAID will dutifully store the bad data the apps send to it).


The above are software and user failures. If you are confident you (or an app) will never make a mistake where you need to rollback to an older version then you simply don't need a backup, and going to a RAID to mitigate potential HW failure is fine. If you are like the rest of us an make mistakes then you probably want actual backups, and to add a RAID in addition to that if down time is very costly for you.