NAS is full quick and can't find the files that fill up the space

I have myCloud mirror EX2 which haas 2 nos. of 2TB drives. RAID is set as RAID 1. It is used for storing the data files for all my 6 computer users in the office to access. All data files are stored in a folder named “Public”. The total size of files inside it is about 20GB. We do not have too many media files such as photos, videos and music stored. Recently, it is reported the NAS drive is full. When I look into the dashboard, the media categories only has about 500MB usage and the “Others” category is about 1.94GB usage. I back the everything to a computer on the network, format the drive and then copy back the back up folder to the NAS. I checked the size of the backup folder in the computer, it is only about 20GB. When I check the NAS, I can see 1.94GB free space. We were back to work,
Yesterday, I got the same error message: “Disk is full”. I checked the dashboard, free space is 0, “Other” category is 1.96TB. We only worked for a day! I check through all settings and don’t find any suspicious settings as our work and requirements are simple. When go to “Shares” and click on the Public folder, it shows the usage is 1.8T but when I copy all to a Windows PC, only 20GB are copied. So what files in the Public folder use up the other 1.6GB? How can I stop this happens again after I remove the public folder and copy back the files?
Thank you for your advice.

I did System Restore and Quick Restore, both do not recover the missing space. I am not doing Full Restore, it started 30 min ago and the Drive 2 LED keep on flashing but the status bar of the message box shows 0% in progress. There is a button in the message box: [switch to quick restore]. What should I do?

Correction: It should be “I am doing Full Restore” in the 1st line. Sorry

Also, Update: I click the Quick Restore. I hope this change won’t hurt the NAS

The status bar is still 0% with Quick Factory Restore title, the Drive2 LED is still flashing in blue. What should I do now.

I think the 0% status is fake as I open another tab in the browser and enter the NAS IP, I am directed to the admin log in page. After logging in, I see the status message box and it is 16% with “Full System Restore” caption. May be the NAS does not update the original message box.

Update as at July 12. I did the Full Restore on the NAS and then recovered the data files. After all is done, 22.22GB is used - that is reasonable. 2 hrs ago, I checked the status of it through myCloud.com, only 2 % is used; 30 min ago, I checked again, it is FULL.
Is something wrong with it on the disks?