If you are trying to upgrade to Windows 10, but keep getting the cyptic error code of 80070002, it could be time to open your box and unplug some cables.
For me, I always saw the 80070002 error code, and Microsoft's official explanation and workaround did not help. Fortunately, this forum post contained the answer I was looking for: Unplug all other hard drives containing a former Windows installation.
Unplug hard drives to get Windows 10 upgrade to behave - TechRepublic