External hard drive problems (Toshiba)

Got a external Toshiba 400gb USB2 drive rigged up to a older HP Pentium 4 machine which *was* acting as a great little Ghostcast server.

Until a few weeks ago.

It is connected to a USB port on a PCI USB2 card, and it is picked up initally. However, if I try to access more than 1 or 2 files the drive LED turns from bright blue to bright red and it won't work again until I switch it off and back on.

Seems to work fine connected to my main PC though?

Any ideas?