USB External Drive problem

Recently I upgraded to an Intel Core2 Quad processor which required a BIOS upgrade.
Everything works fine and Vista flies but a strange issue with my external USB drive has
developed. When I start my computer and check "My Computer", it shows the hard drive but
when I try to open it It tells me the folder is empty. The hard drive is also shown in
the tray as I can "Safely remove hard drive". If I unplug it and I plug it back into the
PC, I get an unknown device message. However, if I shut off the drive and turn it back
on, it's fine. I thought there might be something in CMOS that needs to be changed but I
didin't see anything there that seemed relevant. Anyone got an idea?