Hyper-V VM's Won't Start After October 2023 Windows Updates

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This post was originally published on November 5, 2023
The latest update to this post was made 10 months ago.

Issue

After installation of Windows Updates (Oct 2023) stand-alone, random Hyper-V VM’s fail to start with error:

Synthetic SCSI Controller (ID): failed to power on with error ‘incorrect function’
Attachment <filename.vhdx> failed to open because of error ‘incorrect function’

Event log on host hypervisor may show:

Error 12010 Hyper-V Worker (‘VMNAME‘ Synthetic SCSI Controller (Instance ID <GUID>): Failed to Power on with Error ‘Incorrect function.’ (0x80070001). (Virtual machine ID <GUID>) –AND/OR–
Error 12140 Hyper-V Synthstor: ‘VMNAME‘: Attachment ‘C:\Hyper-V\VMNAME\Virtual Hard Disks\FILENAME.vhdx’ failed to open because of error: ‘Incorrect function.’ (7864368). (Virtual machine ID <GUID>)

Note: Windows Server 2019 and 2022 running Hyper-V seem to be the primarily affected HOST server O/S.

Uninstall the updates for Oct 2023, specifically KB5031364 and restart. This would be considered the official fix (for now). -OR-
Rename the affected .VHDx file(s). Visit the folder with your virtual disks in it. Rename the .VHDX file to something different from what is now. Visit the Hyper-V VM settings menu, under SCSI controller ‘browse’ to the newly renamed file. Apply changes and see if the VM will start. -OR-
Delete the .MRT (modifiable region table) and .RCT (resilient change tracking) file(s) in the virtual hard disks folder of the affected VM. These files are typically present if you use a backup solution, such as Microsoft Data Protection Manager (DPM). This is the quickest and easiest fix and doesn’t require re-starting your host VM server.

A fix is expected to be released in the November 2023 updates, although this is not 100% guaranteed as of the writing of this post.