Boot into a command prompt (Not with the OS loaded, recovery mode in Windows 7; Advanced System Settings -> Restart in Windows 10 - or boot off install media and Shift F10).

Windows 10 Note

Windows 10 Update 1511 Seemed to bork at around 83%. I reverted it and found the profile had lost the start menu icons and was having lots of issues. This may or may not have been related to this method but I suspect it was. The solution was to copy the Users folder back to drive C, perform the upgrade and then move it all again.

Procedure

Copy the files with:-

robocopy /copyall /mir /xj C:\Users D:\Users

Delete the old users folder (or rename it for safekeeping)

And make the symlink (junction) :-

mklink /J C:\Users D:\Users

If the drive mapping is different in the non-os booted system create a hanging link that will map correctly when the 'real' OS is booted.

Update: Seems to work OK with Windows 10 but see the above caveat.