No removable drives

Recently my MultiFlash memory card reader does not enable drives/drive letters in my Win2000 Pro system. In Computer Management/Disk Management, the disk(s) is/are present, but without a drive letter. If I “Add” a letter, I receive an error message which asks for a reboot since”The partition/volume is not enabled”. A reboot most of the time does not provide a drive letter, but another Disk Management “Change Drive letter” process succeeds. Unfortunately the new drive(s) disappear later, generally after a reboot.
The card reader is OK – and works properly on another Win2000 computer, where, without installing anything, the proper driver letters appear. I suspect that some setting (Rigistry?) was changed by either software or Win2000 Service Packs or patches.
Any suggestions?