USB HDD goes missing

I wanna pick some brains. WinXP SP2 usb 250G drive on laptop. Format, label and assign drive letter. Works fine until disconnected or pc re-booted. It then shows up with no drive letter. Interrogation with PQmagic showed the drive (one primary partition) as hidden. Tried to unhide -- no effect. I have recreated the partition once (new disk, not much data, and all replaceable) but the FRED did the same thing again. Tried releoading MS device drivers, search microsoft site (ouch I am desperate) but it still does it. Can anyone help please?
Graeme