Hi there.
I really think the SupraDrives are made by Vortex. If you do think that I'm wrong in this point you can download Scsitools and give them a try. You can find them there: http://www.rsi.pl/paskud/service/scsitool.zip
They absolutely don't want to work together with Vortex-drives.
I don't know whether Ahdi works together with a Vortex hd. You can try to find it on the web.
But of what you write I'd think that the drive is damaged...
If only the 'c' was missing you could put it manually onto the desktop. Do not forget to save your desktop after that.