I had exactly the same problem on an old compaq deskpro 4000
I plugged in a WD 8,4 gig and the pc wouldn´t see it.. the.. when windows booted.. it showed up perfectly fine..
Then.. when I tried accessing the drive under dos.. the OS tells me that there is no such drive or directory..
I´ve never gotten around this problem, but it worked under windows.. so I left it that way..
Let us know if you might have an answer..