I was converting a Windows 2012R2 server from a VMDK to a VHDX, creating a new Hyper-V Gen2 host and struggling to get it to boot. I could mount the drive, read from it from a safe CMD prompt, tried to make sure the correct partition was active using DISKPART, even used BOOTBCD to try but with no luck.
I eventually found that the issue was because I was creating Gen2 Hyper-V machines rather than the Legacy Gen1. Whilst Gen2 will support Win2012R2 they do not support the booting of IDE drives connected as a virtual SCSI controllers can only be recognized after installing Hyper-V Integration Services on the guest OS on the VM. If this is not the case then the guest OS cannot boot from a SCSI disk.
Running up a Gen1 machine and connecting the drive to the IDE booted first time...!
