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BugZero found this defect 2086 days ago.
After successfully restoring a UEFI-based machine running Server 2008 R2 SP1 to a Hyper-V host, the VM fails to boot.
The machine fails to boot due to a machine configuration that conflicts with Hyper-V. For Hyper-V guest Virtual Machines, Server 2008 R2 is only support for use with Generation 1. UEFI-based boot is only supported by the virtual machines using Generation 2.
As Hyper-V only supports Server 2008 R2 running in a Generation 1 VM, and Generation 1 in turn only supports MBR, the only solution to make the restored UEFI-based VM operational is to convert the bootloader from the incompatible UEFI (GPT) to MBR. This process is documented in the Solution section of KB2407.
Windows guest operating system support