Nethermind, i think i solved it by myself. Proxmox Backup Server had nothing to do with this error. It was because of a big copy job which was running on another disk to that time, the problem was, that the folder structure was on this Disk and everytime a new file was moved, there was a little read/write on the Disk, therefore the Disk was active for a short time. Now the copy job is done (5TB) and all disks could spin down...