Mac – My 5TB HD now thinks it’s a 3TB

I'm at a total loss to explain this strange incident. I have a dedicated Mac Mini as a media server to the house, and it has a 5TB external Seagate drive hanging off of it. The drive is partitioned into two partitions, one called "Media" which is 3TB and contains our household movies, TV shows, etc. The other is called "TM iMac" and is a network volume I use to backup my personal workstation (an iMac) via Time Machine. It's been set up this way for years, never had a hitch until yesterday.

Yesterday I noticed that my iMac kept complaining about a problem connecting to a share on the media server. I finally investigated further last night and found that my iMac was failing to connect to the TM iMac network volume. The last successful TM backup was yesterday at 9am, and nothing unusual happened all day. I checked the media server and sure enough, the TM iMac volume was missing. But that's when things got really weird. I ran Disk First Aid and found that, according to it, I don't have a 5TB drive with 2 partitions connected, I have a single 3TB drive with 1 partition – the Media partition. All verify/repairs pass with flying colors.

What the heck has happened here? How has my 5TB drive suddenly turned itself into a 3TB drive? How can DFA think it's anything other than what it is? It would be strange but at least make some sense, if one partition was lost somehow and my 5TB drive reported only a single partition of 3TB in use. But DFA actually says it's a 3TB Seagate drive, not the 5TB that it actually is!

Is this some strange indication of the drive beginning to fail? The drive was purchased new and put in service in March of 2016. Should I nuke the drive, reformat, and restore from backup (I have backups of the Media partition)? Any other ideas on what caused this and what to do about it?

EDIT to include disultil list results:

Media-Server:Volumes media$ diskutil list
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            249.6 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
   4:                  Apple_HFS Time Machine Media      249.5 GB   disk0s4
/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *3.0 TB     disk1
   1:                        EFI EFI                     314.6 MB   disk1s1
   2:                  Apple_HFS Media                   3.0 TB     disk1s2

Please note that "Time Machine Media" is a separate partition on the internal drive of this machine, used for this machine's own local TM backup. It has nothing to do with this question.