I had 3 cases of disappearing Data on Qnap. Not exactly Disappearing techincally, but the user couldn’t see the shared folders in File Mananger.
Your Data Actually is in /share/MD0_DATA ( for older systems) /share/ CACHEDEV2_DATA ( on newer systems).
As long as your MD0_DATA or CACHEDEV2_DATA directories have the data, you are safe.
Data you see in File Manager is a symlink- A Windows Shortcut
Qnap also creates a softlink in your /share directory that points to a directory inside MD0_DATA or CACHEDEV2_DATA. Think of a symlink/softlink as a windows shortcut you create on your desktop for quickly accessing your file.
Deleting Symlink Doesn’t delete the Data . Just that you can’t find Symlink.
This is where the problem is. Symlinks sometimes are deleted automatially ( I don’t know how but I have seen 3 cases where the symlinks were getting deleted). In one case the Firmware got corrupted while update and the link was broken, in other 2 cases I don’t know what caused it.
Now your Data is intact, its in MD0 or Cachedev directory. However you can’t see it in File Manager.
Recreat Symlink and you have your Data Back.