"incorrect super block"
 

This is my fstab entry for my second HD:
/dev/ad2s1 /back ufs rw 0 0
I do "mount /dev/ad2s1" and a message "incorrect super block appears". What does this mean?
I use FreeBSD 5.2.1.

You need a partition on that slice.

You might already have one. Change the fstab to:
/dev/ad2s1a /back ufs rw 0 0

You also want to check this filesystem at boot (you are automatically mounting it) change the last 0 to 2 (or whatever numbers represent your other normal partitions if you changed them from the defaults.