I bought a freecom USB hard drive, on sale at Morgan Computer for £70 to have backups of everything while I was away (and most of my music collection, for later transfer to the Zen micro). It would not work in either of the computers here. I took it back; it worked in the shop, on their computers. On mine it showed up as a connected USB device, but not as a disk drive.
Then I discovered that my little memory stick wasn’t recognised as a disk drive either. This is known good and has worked for years on the other laptop. So it had to be a flaw in the computer. To cut a long google short, there was a confusion of drive letters in the logical drive manager because I have masses of network drives mapped as letters. Assigning the USB drive to something high and unused made it show up at once as a proper disk drive. I offer this tip for the eye of Google, and assistance of passers by.