When seing *lots* of complaints like this, I used to think it is kind of hardware problem.
Until recently I tried to change appearance of drive letters in My Computer using TweakUI.
After this, my USB flash failed to appear in explorer - that never happened before!
However it got a driver letter assigned (in disk manager). Knowing this letter, I was able to open it
from command prompt (start X

From the folder window, I opened up the folders tree pane - and the root of the flash drive
showed there as "zero length string". In the "my computer" view, it was simply not there.
Ater several plugs in-out, the normal behavior restored.
The most weird thing, TweakUI haven't change the drive letter apperance as advertised.
This makes me think, it is a TweakUI bug, and it could affect more users.
I'm using the XP version of TweakUI from MS downloads.
--PA