Microsoft knows its May 2019 update for Windows 10 will wreak havoc on your PC, if there’s a USB drive or SD card plugged into it, so it’s blocking the update from being installed. You’re welcome?
I mean, at least they’re telling us. If you see a very helpful and specific “This PC can’t be upgraded to Windows 10” error, it means the May 2019 update has been blocked because a USB drive or SD card has been detected. I guess it would be too easy to just say that in the error message. Microsoft knows you like a challenge.
Basically, the upcoming update has the potential to reassign drives. So, if your flash drive was originally mounted as drive D, it could end up with E or F by the end of the installation, and that’s less than ideal. Also, sort of more importantly, this can affect internal hard drives as well. Microsoft’s support page for the issue is downplaying that little bit of information by emphasising the USB and SD link.
Since the fix is meant to be simply unplugging said USB drives or SD cards, one wonders what you’re meant to do to stop it borking your internal hard drive? Migrate to another operating system, perhaps?