Do you have another USB port (or a USB hub) that you could plug it into ?
Just that sometimes drivers seem to be loaded for each different port and using a different one might 'kid' the machine into prompting for a new device being found.
Incidentally, it's not that common (in my limited experience with about 6 different memory sticks) for any drivers to be needed (such that you'd get an out of disk space error).
Is it a really large (16 GB+) capacity stick, or one with some special encryption/ extra software on it? Some do have extras and when plugged in appear as two drives, a CD-ROM and a flash drive, where the CD-ROM autoruns to load software to provide options for the flashdrive (such as free software, or encryption facilities, or password protection).
Also, while nearly all USB drives are compatible with both USB v2 and v1.1, I have sometimes known a drive be classed as "unknown device" if plugged in via a hub where power or something else seems not quite right... only option I have found is to use a different USB port if that happens... Have had a few other items (IDE disk controllers for external HD) which finally report "unknown" on every port of every PC, and it was bin time (but admittedly, only cost a fiver to start with, 5 years ago now).