"I need to move several VMs from one hyper-v host (2012) to another host (2012 R2), but some of these VMS have attached a physical drive."
"1. turning off the vm
2. i've create a new vhdx disk as a copy of the physical drive that was attached to my VM
3. i've created a new vm that of which disk is the new disk i've created
The problem is now that the original VM has no physical disk attached to it and ofcourse it doesn't start."
Also you have mentioned :"The problem is now that the original VM has no physical disk attached to it and ofcourse it doesn't start." .
Generally ,the pass-through disk is used for data .
Could you please tell us why the original VM can't start without the pass-through disk ?
The original VM still can not startup when you attach the copied VHDX file?
If the original VM can start up with copied VHDX file ,you may use export/import to migrate it into 2012R2 hyper-v host .