This is a classic problem with Windows,
any version. It will occasionally get it into its head that your drive *might* have problems, and mark it with the hidden dirty bit. The only way to fix it is to reformat your drive -- I've never had any luck clearing it by scanning.
You can also clear the dirty bit by
editing the raw drive with a hex editor. Needless to say, this is involved and risky. Here be dragons.
I have been ignoring this problem on
all of my removable drives/ereaders/etc, for years now -- because I can't be bothered to copy all my files and reformat my drives just for a
Windows-specific idiocy.