Portable USB hard drive not recognised by windows

I have a dell dimension 8300 and recently bought a seagate barracuda portable HDD (750gig). Initially plugged it into my USB2.0 hub which is plugged into the PC - no problems. Worked fine - very fast. Then today had a couple of problems with the powered USB2.0 hub - it kep powering down. Took out the power lead and the other 3 USB connections and just left the HDD plugged into it. PC didnt recognised the drive. Tried a few more things and then plugged the HDD directly into the PC. Still not recognised. Tried plugging it into another machine (laptop) still not recognised. Checked devices on control panel on the dell and all seemed fine (although I do have a large number of USB devices connected to my dell machine via 2 hubs and direct connections - everything from 2 printers, webcams, wireless keyboard, fingerprint reader, scanner etc etc)

Last thing I did was took out the USB2 cable and used Firewire instead. Plugged that direct into the Dell and it was recognised no problem. The problem is that Firewire seems quite a bit slower than USB2.0 and I would like to use the USB2.0 connection from the HDD to the computer instead of firewire.

Any ideas?

Dave