Reformat it anyway, that almost always fixes it.
1. Open up "My Computer" on the desktop, you should see all your drives listed, find the flash drive you have attached, in the example below you see 2GB CRUZER. Right-Click this icon and choose "Format" from the list of options.
2. One the Format screen place a check mark next to Quick Format, and select the drop arrow for the "Allocation Unit Size" and try an alternate size like 8196 - 4096 or lower (*Note - if the only option is "Default Allocation Size", no worries this process should still work).
3. Verify the File system is FAT32, and click the "Start" button to format.
**Here is another drive I have connected to show a few of the Allocation unit sizes you may come across. Again, for the Fit USB - the lower allocation unit size is more likely to work:
I believe the 32GB Kingston USB in my Fit is actually set at 16k file allocation size - works like a charm.