Does this issue occur with all thumb drives, or just one in particular? Try other drives and see if the problem persists.
If the problem only happens with one drive, copy the data off of it to the comp, then format the drive, then copy back the data.
Also, some thumb drives come with the U3 software and U3 partition on them. I've found that product to be a nuisance. I always remove it using the U3 removal tool
. (your drive does not have to be a Sandisk drive)