Ok...here ya go:
in the Terminal (or at the console after you've done command-s):
mount - type this to find out what device the drive you want to do fsck is (i.e. /dev/disk0s9, or something like that)
sudo -s newfs -N /dev/rdiskXsX
(Note the r there - it's important!)
This command is to make a new file system, but with the -N option, it just displays the file system at the given device. At the end of the output it will have a line that says super-block backups (for fsck -b #) - you use one of these for fsck.
Then you would type fsck -b block # /dev/diskXsX and fsck should check the other hard drive fine for it.
Note: if you don't feel like trying out newfs, one of the super-block backups is usually on block 32 on any drive - so you could try fsck -b 32 /dev/diskXsX and it will most likely work.
2nd Note: This won't work if the drive is mounted read/write, it needs to be mounted read-only.