For more than a month I was trying to get an old Windows Server 2012 R2 VM running on fully patched Windows Server 2025 with Hyper-V. In September 2025 I needed to speed up this process so I was searching the internet for a possible solution. In fact something has changed (and deployed via Windows Update) on Windows Server 2025 and/or Hyper-V that causes this problem – I have also went through this thread but adding new SCSI controller did not help..
After removing (in September 2025) update KB5063878 Windows Server 2012 R2 VM booted without problems:
Luka Manojlovic 
Pingback: Inaccessible Boot Device - Windows Server 2012 R2 VMs mit Hyper-V unter Windows Server 2025 booten nicht - Jans Blog
Obstaja tudi situacija, ko ne moreš odstraniti omenjenega KB, ker ne obstaja na sistemu (npr. In-place Upgrade Hosta ipd.). V tem primeru lahko deluje tole:
Vladimir Shebaldenkov
Nov 8, 2025, 10:48 PM
I compared the registry of a VM that boots with one that doesn’t and found some differences.
Then I booted into recovery mode, copied the missing registry keys, and successfully booted the Windows Server 2012 R2 VM on a 2025 Hyper-V host.
reg load HKLM\TempHive c:\Windows\system32\config\SYSTEM
reg copy HKLM\TempHive\ControlSet001\Enum\ACPI\VMBus HKLM\TempHive\ControlSet001\Enum\ACPI\MSFT1000 /s
reg copy HKLM\TempHive\ControlSet001\Enum\ACPI\Hyper_V_Gen_Counter_V1 HKLM\TempHive\ControlSet001\Enum\ACPI\MSFT1002 /s
Vir: Problem booting Windows 2012R2 VM on Windows 2025 Hyper-V – Microsoft Q&A
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Tenx, Damjan!
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