I have a ScanDisk 1T Drive that I could not mount. This is how I fixed it.
1. Open “Disks” application (gnome-disk-utility 3.38.2)
2. Select the device on the left side, then…
3. Click on the gear Icon “Additional partition options”
4. Select “Repair Filesystem” – let it finished (it should say repaired)
5. Now try mounting the USB drive.
If that doesn’t work, you might have to reformat and start from scratch. But check Linux Mints blog for other things to try.