I have a SanDisk Cruzer Micro USB Flash Drive 2GB.
Btw, mine doesn’t look as good as this one anymore – I carry it in my pocket with my keys everywhere I go!
Earlier this week I went to copy a large file to it and it told me it was out of space.

After deleting pretty much everything from the drive I still did not have all of my space back!

Highlighting the files on the drive, I right-clicked and chose “Properties” and was shown a little over 6mb in file – the correct amount. However…

When I highlighted the drive under “My Computer” and right-clicked and chose “Properties” the drive was still reporting an excessive amount of space used! 678mb! WTF?

I searched the ‘net for an answer and many folks with similar problems advocated formatting the drive. Umm… Drastic.
The NON-DRASTIC fix follows!
Go to your “My Computer” and under the “Tools” menu choose “Folder Options”. Under the “View” tab uncheck the box for “Hide protected operating system files”. Don’t forget to reselect this when you’re all done.

Now when you look at the files on your flash drive, you will see a “Recycled” trash-bin icon.
Right-click on the trash-bin icon and choose “Delete”.

Yes. Yes, you do want to delete this pesky little critter.

Don’t worry, it’s not going to hurt anything. I did this on mine and all is well.

Checking the space on the flash drive again, all of my drive space was back!
Far as I can tell… not disconnecting your flash drive correctly before removing it could cause this. Or using it between a PC and a MAC was a theory on the ‘net. Or just gremlins. You fed them after midnight didn’t you?