Windows Explorer does NOT find devices (but are working USB device

I have 2 USB "Storage Vaults" (external hard drives) which I have used for
several months with no problem (Windows XP-Pro). 5 days ago, they suddenly
stopped appearing as visible drives using Windows Explorer. They
both,however, DO show up in Control panel/Systems . . and both are indicated
to be "working properly." But, Windows Explorer does NOT FIND THEM, so I have
no way to browse the contents, delete files, etc. I've tried EVERYTHING I can
think of . . . but WIndows Explorer still doesn't find these 2 drives. What
can I do? Thanks.