Hi,
thanks for the help, the re-measure button did the job, now the disk is shown with the correct size.
Don't know why I was that blind, was looking for exactly this option but only checked the top right troubleshooting menu with cheching duplication etc...
File size and size on disc is nearly the same, no filesystem compression or dedup enabled.
Btw the Onedrive sync has some serious problems when you remove/add disks, looks like every file which will be transfered to another disk is triggering the Onedrive check.
So Onedrive is syncing like hell and sucking CPU usage all the time.
Maybe that was the problem that the pool was showing more file size than really on the disk.