I have a brand-new Sandisk 64 GB Ultra thumb drive.
When I plug this thumb drive into a USB port on my Windows 10 computer, the LED on it lights up, the control panel recognizes the drive as "Ultra Backup", but File Explorer does not recognize it. File Explorer on my Windows XP machine also does not recognize the drive. That tells me this is NOT a registry problem. Other thumb drives I have work fine, so I think this is probably not a driver problem (contrary to what a Sandisk tech support person told me).
I didn't find the right solution from the Internet.