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External is a Seagate 2TB Backup Plus Desktop Drive for Mac. I'm running 10.11.6 on a Mac Mini. It's connected by FW800. Disk Utility First Aid for just that volume stalls after the "Invalid Extent Entry" note.

I ran Disk Utility on the entire external drive (all volumes). Result: [All checks seem good, then...] "Problems were found with the partition map which might prevent booting. Operation successful."

Other 3 volumes on that Seagate seem to work fine. One of those volumes has secondary Time Machine backups.

After restart, Mac without prompting says: "OS X can’t repair the disk “Backup.” You can still open or copy files on the disk, but you can’t save changes to files on the disk. Back up the disk and reformat it as soon as you can."

I assume I should erase that Backup volume but my concern is I'm afraid erasing that Backup volume might screw with the other volumes, and my only other TM backup, which is on another volume of the same external HD.

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The invalid extent entry error basically means the directory on your drive is corrupted. In other words the index of what files are stored and where they're stored is all mucked up.

This can be repaired but not without risk. You basically have two options if you still want to keep using this drive:

  1. Use a third party disk utility that can repair the corrupted directory (e.g. DiskWarrior, Techtool Pro, etc)
  2. Reformat the partition

Note: I have no affiliation whatsoever with either DiskWarrior or Techtool Pro.

Regardless of the option you choose I would still recommend manually backing up any files you can still access to another drive. Obviously this is a must do if you opt for the second option.

As to your concern about accidentally reformatting the other partitions, this shouldn't be a problem as long as you're only selecting the particular partition (or volume) with the error.

More specifically:

  • launch Disk Utility
  • select the partition in the sidebar
  • click the erase tab
  • make sure the format in the drop-down menu is the one you want
  • give it a name
  • click on Erase
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  • Thanks again, MM. I'll try #2 tomorrow and report back. Might go buy another HD and backup first. Since you're familiar with my recent computer problems...it's weird, my Mac seems to be screwing up drives recently. Another external died when I tried to put OS 10.11 on it from Recovery mode.
    – ScotttDC
    Commented Mar 5, 2017 at 0:37
  • I think buying another HDD and doing a backup is an excellent idea. It's what I would do. All the best!
    – Monomeeth ♦
    Commented Mar 5, 2017 at 0:38
  • MM, I have another old HD lying around. It's a Seagate and I think it just has a bad mini USB port. I've ordered an adapter to use the alternative port, an eSATAp ... So I'm gonna try to back up to that drive, which is cheaper than buying a new one. Then I'll erase just the Backup volume or reformat the whole drive. Which is the better/optimal option? Is reformatting the whole thing overkill?
    – ScotttDC
    Commented Mar 7, 2017 at 23:12
  • Reformatting the whole thing is probably overkill, especially since you'd want to be 100% sure you have a good backup of a lot more data then you would if it was just one partition. Please make sure you are absolutely satisfied that the old Seagate HD is still good before risking the format.
    – Monomeeth ♦
    Commented Mar 7, 2017 at 23:21

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