Hello, the first step is to disconnect all other devices from the Mac and reconnect the drive again. If this doesn't work, it is mostly possible that there is something wrong with your USB drive.
You'd better recover and backup the data with a professional tool then fix the USB drive. It's highly recommended to utilize iBoysoft Data Recovery for Mac, an Apple-trusted and reliable software to recover all your data at once.
- Recover lost data from the USB drive on Mac first with iBoysoft Data Recovery for Mac. Free download, install, and open it, then select your USB drive in the left sidebar and click Search for Lost Data. After scanning, preview and choose your wanted files to save to another place.
- Show hidden files on flash drives. In the Finder, press Command, Shift, and period.
- Run First Aid in the Disk Utility. Launchpad > Others > Disk Utility, select your USB drive and click the First Aid on the top bar. Then click Run and repair the drive errors.
- Reformat the USB drive. This step will erase all the data, please recover and backup with iBoysoft Data Recovery for Mac. Launchpad > Others > Disk Utility, select your USB drive and click the Erase on the top bar. Choose the format then click Erase.
Also read: Can't See Files on the USB Drive Mac, How to Fix?