Firstly, go to Control Panel > Appearance > Folder Options and click the View tab. Check the box next to Show hidden files, folders, or drives, click OK and try to open the files. If you can't find them, press Windows + R key combination, type devmgmt.msc and click OK. Expand the Universal Serial Bus controllers list, right-click on the driver and select Uninstall. Reboot your computer, connect the USB flash drive, press Scan for hardware changes icon from Device Manager and wait for Windows to retrieve the drivers. Also, update Windows Movie Maker to the latest version. If you still have issues, I suggest you contact Microsoft Support team.