Some damaged disks do not show up with Disk Utility and DiskWarrior, and may hang and crash such repair utilities. Yet, there is a trick to repair such broken disks using Disk Arbitrator.
1. Open Disk Arbitrator and select "Activated - Block mounts." This prevents the damaged disk from crashing the repair utilities and allows such disks to be displayed by these utilities.
2. Connect the damaged disk.
3. Open Disk Utility or DiskWarrior. The damaged disk should show now and it should not crash the repair utility.
4. Deactivate Disk Arbitrator (uncheck the Activated box). This allows the repair utility to handle the damaged disk properly (mounting it once repaired).
5. Run Disk Utility or DiskWarrior.
Then, the utility should repair the disk, or at least, mount a Preview disk (DiskWarrior), allowing you to inspect and even copy or backup some files or the full disk, reformat it and restore it.
[kirkmc adds: Well, I guess I'm fortunate that I can't test this. If anyone has any severely damaged hard disks around, and wants to try, feel free to post in the comments whether or not this works.]
Mac OS X Hints