Very interesting thread!
Agree with the preceding post...I believe the move should be accomplished as--
make an ftmb backup including the media,
move that file to your D: folder and
open it there,
then change the file locations in options to reflect the new location as well (for future actions).
Recognize that the program itself will still be running in C: and potentially there may be issues with that (pgm in C: and data in D:). That depends on how well ancestry wrote the code to handle such.