If you have a Microsoft HyperV Checkpoint / Snapshot that has come disconnected from the Hyper V Manager console, then you need to manually merge the VHDX and AVHDX files. Fortunately, this not generally very difficult:
- Shut Down the VM in question – This is not technically required but it will be faster and will reduce the likelyhood of data loss
- In HYPER V MANAGER, click EDIT DISK from the actions pane on the right
- BROWSE to the .AVHDX file for your VM (not the .VHDX) and click NEXT
- Select MERGE and click NEXT
- Select TO THE PARENT VIRTUAL DISK and click NEXT then FINISH
If this fails with Failed To Merge The Virtual Disk 0x80070032 as shown in the screen shot to the right, go through the process again but this time select TO A NEW VIRTUAL HARD DISK at the end. Once that is done, you will have to delete the old VM and recreate it pointing to this merged VHDX. If you try to just change the VM’s SETTINGS > VIRTUAL HARD DISK to point to the new VHDX you will see it errors out with a MERGES PENDING message.
Remember that you can do all of this using PowerShell if you want. See THIS Microsoft Merge VHDX article for details.