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.