Happy to offer you some help, there are some fixes you can try, but pay attention some may cause data loss on your USB. So you'd better utilize a powerful data recovery software like iBoysoft Data Recovery for Mac to recover all your data because you can't use it on another Mac.
- Update your USB drive: Check it in Apple > Settings > General > Software Update.
- Click the option ignore under the USB in the Disk Utility: Choose your USB and click on First Aid in the top menu, then select Run and wait for the repair.
- Repair it with Mac Terminal: Launchpad > Others > Terminal, then type diskutil list then enter diskutil verifyDisk /dev/disk identifier. After you see "repairing the partition map might erase the disk", type y.
All of them can't help? Sadly, you have to recover and back up your data with iBoysoft Data Recovery for Mac and then reformat your USB.
Also read: How to Fix Unreadable USB Flash Drives on Mac without Data Loss?
