I had the same problem last summer. The manufacturer of the equipment I was working on said they never put a password on the drive, and the customer said they didn't. What now? The most frustrating part was I used to know the back door password but I didn't keep a record of it.
Call you local Allen-Bradley rep. I have found they don't tend to guard the passwords on drives as closely as other passwords such as PLC passwords. If they don't want to give you the back door password, ask them to come out and enter it in while you look the other way (wink, wink)! Its worth a try.
Sorry, it wouldn't be cool to post the password here.