the grandparents are old and senile and need to be able to save things to the default C drive
Think sideways my friend.

Leave the old drive in attach the new as a slave.
Put a Shortcut on the desktop to the new drive and call it "C:" or "My Stuff" or something.
Move all their old stuff into it.
In the real C: where they were storing all their stuff put another shortcut to the new "C:" or "My stuff" whatever you call it.

They'll get it quick enough if every sign points them to it. They still go to the old location but all they will find is a link to the new.