Sorry it took so long for me to try things.

What can I do if Knoppix won't mount it? (Please note that knoppix won't even pretend it's a disk.)

I would still suggest the things I did before, first try a new usb cable and if that doesn't work then it's pretty much down to pulling the drive out of the case and installing it in your computer and seeing if that works. If that doesn't work then about the only thing I can think of that's left is professional recovery which could get expensive depending on how much you have to recover.

Mmm, you said the boot record is broken. Last month the DBR of my pc was broken and data also lost. I fixboot it by EAUSUS Partition Table Doctor to recover my data. Amazing software! You could download the demo version to check if it can recover. I think it is helpful. ptdd.com

Hi, this is the USB RAW. I met this situation last month and there is no file in it. I used software named Data recovery wizard to recover it. From this program I can see the USB drive and I recovered by complete recovery. That’s great! My countless pictures, songs, work and personal files were recovered completely. Maybe it will also help you.