First, I would rule out anything else but a bad drive. Going on other threads I've read on the subject, you might want to reformat the drive and then recopy the GBSYSTEM folder with the Rockbox changes, and of course the .rockbox folder. If you cannot get it to USB mode, a trick to do that is to disassemble the F20, disconnect the hard drive, connect the F20 to your PC, power it up (so it goes into USB mode) and then connect the hard drive (it will work, as the drive is designed to be hot-swappable). Then, format and recopy the files. Reference site to disassemble is here:
http://www.angelfire.com/bug2/gigabeat/If that doesn't work, or if you can hear the "click of death", then heres's what I know about drive upgrades (reference:
http://www.rockbox.org/twiki/bin/view/Main/HardDriveReplacement). Without making any ribbon alterations, you need a 50 pin 5mm 1.8' drive. From the chart, it looks like you can go up to a 40GB drive. However, it might be possible to change the ribbon from a 50 pin to a ZIF connector, which would up the size to 80GB (the MK8022GAA). I have not read anyone actually doing this, but I'm waiting to test this, myself. btw, the only reason you can't use an 8mm drive is because of your backplate - if you can find a broken F40/F60 on the cheap, you can swap out both the backplate, and the drive (I can attest to that, as I've done it).