I guess it's simple and you don't need fstab necessarily...
first, plug the HD in. it becomes /dev/hdd if I read your post right, so next we'll simply mount it. create a mountpoint to your /mnt (be root):
the next step is to mount the device. you can add an fstab entry if you wish (if you want to access the disk afterwards simply with one short mount command) but it's not necessary:
mount -t ext2 /dev/hdd /mnt/backup
if ext2 is the filesystem on the HD you want to mount. you can try to mount without the "-t ext2" option too...now if all went well, you can access all the files in /mnt/backup that are in the HD. then simply copy them from /mnt/backup to some other place on your other HD or burn them to cds or something...after you're done,
and you're done. or then don't umount, but format the disk right away