A backup product’s “boot from backup” feature stopped working on one of my Hyper-V hosts. It restores a disk image, attaches it to a temporary VM and boots it, and the attach step failed every time with an unhelpful error. The vendor’s first answer was to reinstall their agent. Before I did that, I wanted to know whether Hyper-V itself could attach a physical disk on that host. It could not, and the reason had nothing to do with the backup software.
A repro that leaves the vendor out of it
The feature works by offlining a disk and attaching it to a VM as a pass-through disk. You can do the same thing yourself with a throwaway VHD, which makes a clean test that cleans up after itself.
$vhd = "C:\temp\passthrough-test.vhdx"
New-VHD -Path $vhd -SizeBytes 1GB -Dynamic | Out-Null
$disk = Mount-VHD -Path $vhd -PassThru | Get-Disk
Set-Disk -Number $disk.Number -IsOffline $true
New-VM -Name "pt-test" -MemoryStartupBytes 512MB -Generation 2 | Out-Null
Add-VMHardDiskDrive -VMName "pt-test" -DiskNumber $disk.Number # this is the step that fails
Remove-VM -Name "pt-test" -Force
Dismount-VHD -Path $vhd
Remove-Item $vhd
On a healthy host the Add-VMHardDiskDrive -DiskNumber line just works. On mine it failed with the same error the backup product reported. That is the moment the support case changes from “your software is broken” to “my host is broken”, which is a much faster conversation.
Where Hyper-V keeps its physical disks
Hyper-V tracks disks it can pass through in a storage pool. Checking that pool showed it was empty, even though the offline disk was sitting right there in Disk Management.
Get-StoragePool -IsPrimordial $true | Get-PhysicalDisk
Something between the disk stack and Hyper-V was claiming the disk first.
The culprit: Storage Replica
Years earlier someone had installed the Storage Replica feature on this host while evaluating it. It was never configured and never used, but its filter driver was still loaded and still sitting in the disk stack. Removing the feature and rebooting fixed the attach immediately, and the backup product’s boot-from-backup worked again.
Get-WindowsFeature Storage-Replica
Uninstall-WindowsFeature Storage-Replica -Restart
If you do not have Storage Replica installed and see the same symptom, there is also a documented regression in Windows Server 2022 that affects pass-through attach, so check your patch level before going further.
Two things worth keeping
First, the one-liner. I keep it in my notes now, and it has already saved a second support case. Second, the habit: when a vendor feature fails, find the operating system call underneath it and run that on its own. If it fails too, you have your answer and the vendor has nothing to fix.