Hello
Thank you for your question and reaching out.
From the error it looks its being used or merging is still in process.
- Try SYSTEM account full rights to the .avhdx files
icacls "full-path-to-vhd-or-vhdx-file" /grant "NT VIRTUAL MACHINE\Virtual-Machine-ID":(F)
- Disable any Back of Antivirus program for temporary purpose.
- Restart Hyper-V and VMMS services
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