USB devices can be flaky and there's no end to stories like yours. I've had a couple, however, that were resolved by simply going into the hardware screen, removing the device, restarting the machine and letting Windows redetect it. It should have retained the driver and will reload it. Now, it does sound as if your device needs more power than most. Such could be the root cause of your issues. What XP will detect is the controller for the drive and the controller will then configure the drive itself. At this time, Windows should see the drive. Can you tell if controller is seen but not the drive? These devices need a bit more current than most others and the lack of power from the 5v to the USB header might be a clue.
Sorry - aware that several previous discussions have tried to deal with similar issues....but the issues vary every time it seems!
I have a Fujitsu (I think - nothing to tell me, and it was given to me new by another person) Mobile DIsk external data storage USB device, 40 Gb. Several days ago my computer (with XP SP2)said it did not recognise it. Previously it did. I have tried the device on another computer and it installs immediately and functions, so there is nothing wrong with the device, which powers from the computer directly. It powers up on mine, and everything indicates that the USB port is ok since I can plug in other devices (eg flash).
Basic question - how does one resolve this kind of problem? I only have a recovery disk for the computer, not a separate set of XP cds, and I hesitate to go all the way back to scratch! If I am correct, this is not a driver issue. Then what kind of isse is it? Would going back to a restore point succeed? I tend to think not. So.....?