27" iMac (late 2012) running 10.14.6 Mojave
An external drive (a Porsche Design 3TB USB drive) that was my TimeMachine drive went off-line, and there was an alert to not disconnect a drive without Ejecting it first (which I always follow). I disconnected the now-unmounted drive, powered it down, re-connected it, and re-started it. DiskWarrior (4.4) reports a hardware failure (-36, 2747) [I don't know if that's a number in the thousands, or two separate numbers, one negative and one positive]. ALSOFT's site says the -36 is a hardware error — bad, obviously.
MacOS's DiskUtility reports no error on it, and it mounted, I started FirstAid and FirstAid is still running. No details in other than:
Repairing file system.
Volume was successfully unmounted.
Performing fsck_hfs -fy -x /dev/rdiskNsM
Checking Journaled HFS Plus volume.
Detected a case-sensitive volume.
Checking extents overflow file.
Checking catalog file.
Checking multi-linked files.
Checking catalog hierarchy.
Everything's either gone, or some of it's still there, so I'll let the First Aid run overnight.