No you have not made things clear at all. You start out saying that you deleted the Plex folder in 'shared folders' but you didn't get your 800GB back.
Then you say you found the folder that contains the 800GB but it doesn't exist, how can that be, that doesn't make any sense at all?
Have you run an Extended SMART test in Storage Manager->HDD/SSD->SMART Test on both of your drives?
Are they healthy, no failed sectors (bad blocks) or any other issues?
If your drives are healthy, nothing will happen and the drives will be marked normal, but if they are faulty at all, then you might get a report.
Note: if your drive/s suffered bad blocks, the drives may have already mapped the bad blocks 'out of service' and the only way to tell is by checking the reported size of the drive in Storage Manager->HDD/SSD.
Check the drive manufacturers website to see what the formatted capacity of your drives should be. If that number is not close to what is reported in Storage Manager->HDD/SSD then you will need to replace the damaged drive/s.
You should also open Storage Manager->volume->manage->start data scrubbing.
To find the Plex folder using SSH use CD /volume1 then ls (that is an elle + ess in case you are wondering) to list the directory. You should see the Plex folder in the list, but remember, you stated that you deleted the Plex folder, so it may not exist unless you have re-installed Plex?