many thanks for your answer. Yes, these unused sectors would have been filtered.
But the idea you gave me raises more questions, it is not clear how the workflow would be.
Create empty VHD.
Copy MBR to VHD (stream) - Do I need to partition it anyway, or is it defined via MBR? (Diskpart -rescan)
Make VSS Snapshot of physical HD partition.
Copy Bootsector of partition to VHD partition as stream.
Do a file copy (all) from VSS Snapshot to VHD partition.
Do the same next partition or finish.
Did I understand correctly?