If I use it for backup, doesn't it make sense to be able to boot from it? to just swap sata cables and bring it up?
You can setup a live, hot backup machine, on the second drive. The setup is fairly complex but pays huge dividends in both redundancy and learning.
1. Setup two disks with identical partitions
2. install distro on disk #1
3. configure samba share and backup important files to this
4. mount partitions from disk #2
5. create boot files on /boot partition for disk #2
6. rsync complete system to mounted partitions on disk #2 (except /boot information, /proc, /sys, /tmp, etc. since these are virtual partitions)
7. when/if you lose the primary system you simply boot into the second system and have a mirror image
8. this can be done with any system setup, web server, database server, email server, etc.
If you do this on two drives the redundancy is reduced but does protect against drive failure. The real payoff is when this is setup between two live machines.