Rebooting the Windows-Server solved the problem. 
From SnapDrive created LUN's should be never changed with other tools like OnCommand System Manager or windows disk management.
These LUN's have have a special disk geometry from SnapDrive.
Example: Resizing a Snapdrive-LUN with OnCommand System Manager create huge "busy" snapshots in a Snapmirror-Relationship, that cannot be deleted!