Repair External Drive on MacOs

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