I corrupted an external 4TB Seagate Backup Plus drive. Then, it wouldn't mount. It has a number of old iPhoto libraries that I was consolidating. The drive would not mount in Finder or Disk Utility. I dreaded paying for some overhyped file recovery software, but I was getting nervous. Luckily, I added "fsck" to my search terms and found a good solution, already in the MacOS. % diskutil verifyDisk disk6 Started partition map verification on disk6 Checking prerequisites Checking the partition list Checking the partition map size Checking for an EFI system partition Checking the EFI system partition's size Checking the EFI system partition's file system Checking the EFI system partition's folder content Checking all HFS data partition loader spaces Checking booter partitions Checking Core Storage Physical Volume partitions The partition map appears to be OK Finished partition map verification on disk6 % diskutil verifyVolume disk6s1 Started file system verification on disk6s1 (EFI) Verifying file system Volume is already unmounted Performing fsck_msdos -n /dev/rdisk6s1 ** /dev/rdisk6s1 ** Phase 1 - Preparing FAT ** Phase 2 - Checking Directories ** Phase 3 - Checking for Orphan Clusters 2 files, 201628 KiB free (403256 clusters) File system check exit code is 0 Restoring the original state found as unmounted Finished file system verification on disk6s1 (EFI) % diskutil verifyVolume disk6s2 Started file system verification on disk6s2 (Media) Verifying file system Volume is already unmounted Performing fsck_exfat -n -x /dev/rdisk6s2 File system check exit code is 1 Restoring the original state found as unmounted Error: -69845: File system verify or repair failed Underlying error: 1 % diskutil repairVolume disk6s2 Started file system repair on disk6s2 (Media) Repairing file system Volume is already unmounted Performing fsck_exfat -y -x /dev/rdisk6s2 File system check exit code is 1 Restoring the original state found as unmounted Error: -69845: File system verify or repair failed Underlying error: 1 This was not looking good. It was sunny out, and we needed some exercise, so we went outside and cut shitake logs for a few hours. When I returned, I tried again, and got lucky! % diskutil repairVolume disk6s2 Started file system repair on disk6s2 (Media) Repairing file system Volume was successfully unmounted Performing fsck_exfat -y -x /dev/rdisk6s2 Checking volume Checking main boot region Checking system files Volume name is Media Checking upper case translation table Checking file system hierarchy Checking active bitmap Rechecking main boot region Rechecking alternate boot region The volume Media appears to be OK File system check exit code is 0 Restoring the original state found as mounted Finished file system repair on disk6s2 (Media) The drive mounted well during the repair, and I am back to work. h/t: comment #2 at https://superuser.com/questions/503759/how-to-run-fsck-on-an-external-drive-with-os-x |