Hi TerraMaster Support team,
you really did a good job on our online-session. Thank you for solving the issue!
It must have happened during a USB Copy backup job that the USB drive became disconnected. In that case USB Copy is backupping the rest of the not-copied-so-far and partially-copied files to sub folders of /root with the name of the corresponding backup job,
e.g. if the backup job EMailBackupJob gets interrupted by a sudden detachment of the external USB drive then all files that haven't copied yet to the USB drive target are redirected/copied to /root/EMailBackupJob/ instead.
Maybe those in the cache or which were actually copying during detachment event, which have been (partially) copied to the external USB drive are going to raise the above mentioned warning, i.e.
1. file A.mp3 was partially copied to USB drive
2. while the connection to the USB drive was lost
3. TNAS copies all residual files to /root/EMailBackupJob/
4. A warning comes up
5. When reconnecting to USB drive TNAS checks which files from /root/EMailBackupJob/ are also on USB drive
6. Those files (A.mp3) are listed in the Warning dialog under "More"
Thus one needs to delete those USB backup Job folders under /root or move them to another directory and then the warning has gone.
Since there are no files under /root any more comparison between /root and USB drive will no longer find any files any more, so warning will not reappear when copying the same files again to USB drive.
I hope I have described the cause and solution of the issue correctly
So thank you so very much for your great support, TerraMaster support team. I really appreciate your support and I am happy having bought that NAS!