Volume Rebuild not working

I have a 1TB external drive that I've been using for Time Machine backups (Mac OS 10.10.5). I had run out of room on my internal SSD drive and moved my Photos Library to that same external drive. I verified that the photos copied over successfully and could see them in the Photos App. I then deleted the Photos Library on the internal drive. The first time I tried to remount the external drive, it would not mount.

In Tech Tool Pro the external drive shows in the list, but the name is grayed out--indicating that Tech Tool knows it's there but can't mount it. I run "Volume Structures" under "Tests". It only goes through the first stage, with a red bullet next to "Journaled HFS Plus Volume". It then reports "Validation Error Encountered". I then go to "Volume Rebuild" under "Tools". I sometimes get a red bullet next to "Prechecking Structures", sometimes a green bullet, sometimes no bullet. Then it always skips the next three bullets, gives a green bullet next to "Replacing Directory Structures" and "Rechecking Structures". It reports back that the Volume Rebuild Completed and "No Rebuild Errors Encountered". But, the volume remains grayed out and unmounted. I can go back and run the "Volume Structures" test, but get the same result as the first time.

I know the data must still be on that external drive--please, how do I get to it?