Problem migration Hyper-V to Nebula

The task is to migrate a Hyper-V version v2 VM to Nebula
After successful conversion:
qemu-img convert -f vhdx -O raw system01.vhdx system01.raw
The virtual machine writes at startup that there is no boot device
What is completed
1.Before the conversion, virto-win-0.1.215 drivers are installed on the Windows Virtual machine
2.When adding the image, I changed the virt/SATA/IDE buses - this did not give results

What is strange is that if I use a new Windows test VM, the migration is successful and everything works. A problem with a VM in a production environment.

What could be the problem?