I was having a problem with my Surface Pro 3 after the Win10 upgrade, so I decided to revert it back to Win8. I cloned the drive to an external drive using Acronis and then restored the Surface to factory defaults. That worked, and I'm back to Windows 8.1 now, but now I can't access the external drive (it asks for a Bitlocker key). Is there anyway I can get the key back somehow? Is it related to the Surface itself?