From: Sunnyvale, CA (Unite
I used to repair those drives, or train those who did the repair.
You need special tools, etc...And if the Bios was over written, as it sounds like it was, then only Yamaha can resolve the issue.
I do not work for them anymore of course, so I do not have access to the needed tools.
Sorry, but the only thing you can do is reflash the firmware on the recorder. If that failed to resolve it, it would be cheaper to purchase a new drive, than what it would cost Yamaha to repair the older models.