Try these methods when you can't see files on the USB drive on your Mac: unhide files on Mac, repair the USB drive, reformat the USB flash drive, etc. If you need to recover disappearing files from the USB flash drive, iBoysoft Data Recovery, the professional data recovery software may help.
It seems that the connection between the USB drive and your Mac is poor given that the USB drive may don't show up in Finder and Desktop at the beginning. Check the USB port, USB cable, USB adapter and hub, and anything involved in the connection when you can't see files on the USB drive on Mac.
Then, try to unhide files on Mac. You can use the keyboard shortcut Shift-Command-Period(.), and Terminal commands defaults write com.apple.finder AppleShowAllFiles TRUE and killall Finder. In case, these files still won't show up, the USB drive could be corrupted.
The next step is to repair corrupted USB flash drive on Mac. Disk Utility, the macOS built-in tool enables you to run First Aid to check and repair disks on Mac. After the process completes, check whether those files on your USB drive are visible.
If the USB drive is damaged seriously, First Aid won't repair it. The last resort you can try is to recover data from it and reformat the drive to make the USB drive usable again. Download and try iBoysoft Data Recovery for Mac to recover data from the USB drive, then reformat the USB flash drive in Disk Utility.