Your C and D Drives are probably on the same Hard drive, they are just partitioned Drives.
Microsoft would like you to use an external simply to make sure you don't accidentally overwrite a partition, thus wiping out everything you saved.
You can try your D Drive and see what happens, it should work,be very careful not to wipe the whole hard drive if it is just a single drive.
Maybe someone else who has tried it will reply. I have only done complete wipes or dual booting clean installs no upgrades yet.