[Albert Lionheart]

A somewhat grumpy PC with error messages popping up all over the place and notes from MS saying that it was down to a problem with the anti-virus turned out to have a bad lump of memory - removing it cleared all the problems except one really irritating failure to read any external USB drive.
Looking at the list of drives I thought there were an awful lot of mapped drives but not my business to work out why this thing had to connect to so many other machines' drivers.
Anyway, I uninstalled all the USB devices and let the system put them back, which it did without any problems. But still no access to the workshops USB flash drive - which worked on other machines. It seemed to read it but the list of mapped drives just flickered.
Gawping at the list of mapped drives, I noticed that it had used every single letter including drive B. How really interesting I thought and turned it all off to go an post a question on this forum - just as well I stopped to make a cup of coffee on the way as it suddenly hit me - how can the system allocate a drive letter to the USB drive when there are none left? It can't so forget the coffee and back to the machine - disconnect drive Z: and reboot: stick the USB drive up it's orifice - and there it was all the time!
Moral - sometimes the blindingly obvious will stare you in the face for some time before you see it.