This is from llamma.com with a few alterations to fit your problem.
1. Connect the hard drive as primary master (remove the jumper)
2. Create a bootable floppy with ATAPWD.EXE on it.
3. Boot from the floppy and launch ATAPWD.EXE select the drive and press enter.
4. Select ERASE PREPARE and then ERASE UNIT.
5. Enter the master password: XBOXSCENE
6. It will give you a Busy error, that means it is erasing the drive so please dont turn off the computer.
7. After 1 hour press F5 to refresh the list and you will see that the drive has become unlocked
8. If it is not unlocked wait another 30 mins- 1 hour.
This is, if your master was set to XBOXSCENE
Also this not only unlocks the drive is deleted all the data on the drive so make sure your stock is available to clone.