After a reboot today of the server I got the message above.

Cache drive seems to be only half full based on the report on the Main tab.

Error message:

Nov 28 17:54:51 Tower kernel: BTRFS error (device sdg1): parent transid verify failed on 495321088 wanted 1852238 found 1852237
Nov 28 17:54:51 Tower kernel: BTRFS: error (device sdg1) in __btrfs_free_extent:6989: errno=-5 IO failure
Nov 28 17:54:51 Tower kernel: BTRFS info (device sdg1): forced readonly
Nov 28 17:54:51 Tower kernel: BTRFS: error (device sdg1) in btrfs_run_delayed_refs:3009: errno=-5 IO failure
Nov 28 17:54:51 Tower shfs: error: shfs_rename: Read-only file system (30): rename: /mnt/cache/appdata/plex/Library/Application Support/Plex Media Server/Logs/PMS Plugin Logs/com.plexapp.agents.subzero.log.4 /mnt/cache/appdata/plex/Library/Application Support/Plex Media Server/Logs/PMS Plugin Logs/com.plexapp.agents.subzero.log.5
Nov 28 17:54:51 Tower kernel: BTRFS error (device sdg1): pending csums is 217088

Making a copy of the cache drive right now and will try next:

1. reset the cache drive cables

2. reformat the cache drive if above fails.

tower-diagnostics-20171128-1822.zip

22 minutes ago, soana said:

parent transid verify failed on 495321088 wanted 1852238

 

That's usually fatal with btrfs, copy everything and re-format, don't stop the array before copying everything as it can go unmountable, though you could use btrfs restore in that case.

Thanks for the "glove" John!

The cables did not seem to have an influence so I moved to reformatting the cache drive, after about 10 hours of file transferring I'm back in business. 

Plex folder is a monster took me hours to transfer back and forth.