Parallels Desktop displays the following error message when you are trying to compress virtual machine's hard disk:
“The disk cannot be compressed because it is used by the "*VM-name*" virtual machine that is in Safe mode or Undo Disks mode, or has snapshots”
- Virtual machine's hard disk has snapshots
- Rollback mode option is enabled
- Virtual machine hard disk is corrupted
- Virtual machine hard disk has hidden partitions or unallocated space
- Virtual Machine was imported from Boot Camp
Try to compress virtual hard disk after every single resolution step you take:
1. Make sure the virtual hard disk does not have Snapshots: go to Actions -> Manage Snapshots.. and delete all snapshots.
2. Go to the virtual machine configuration > Security -> disable Rollback Mode option if enabled.
3. If your virtual machine was migrated from a real PC, make sure it does not have unallocated space. You may check it in Windows Start menu > Control Panel > Administrative Tool > Disk Management