Disappearing drive letters in XP

I have a USB connected drive to which I had assigned drive letter F. For some reason now, each time I plug it in, the drive letter increments by one (F to G and so on) and I cannot reassign any drive letter it has used before: in other words when I go to disk management the only letters available are those above the last letter it used. I have never encountered this before.

This morning I installed a Linksys router connected to an ethernet port on the PC. The router works fine - I can hardly believe this is the cause. There is no network drive as such and no other computer is connected to the router.

This is getting desperate, since I plug in a card reader too and it's having the same effect - I will actually run out of drive letters soon.

 

Do you use the remove hardware safely wizard when you remove your USB connected drive?