This solved the same issue for me. Spent 4 hours with a 3120L 0046 drive. It would go into mode b but my 6421 sata controller would not recognize the drive and when windows was booted with the 3120 plugged in I got the device manager failure to load drivers for the 6421 pci card.
On the other hand I got my bios to recognize the drive when plugged into my ECS 848P-A mother boards onboard SATA controller. Windows still would not see the drive.
The fix below worked and the drive flashed perfectly with the new 1.4 firmware. This fix was used with the 3120 plugged directly into my SATA controller on my mother board. I have yet to get the 6421 to work with Hitachi drives.
QUOTE(shadowth @ Jul 22 2007, 03:54 PM)

It may be weird but, try this:
When your drive is off, manually eject it. Turn your 360 and PC on, run slax, reboot PC and go to Windows. DO NOT open the tray at any moment.
I had a problem with a Hitachi that I solved this way.
Thanks Shadowth