You cannot change the location of the built in storage calendar
directly, but you can put your whole profile into a different folder.
If the plan is to move relevant data to the D: drive then I'd suggest
moving C:\Users to D:\Users, because then you can delete the C: drive
without needing to backup your documents separately. There are some
extra hoops involved, please search the internet for tutorials.
If you don't want to do that you can move just the Thunderbird profile,
see
http://kb.mozillazine.org/Moving_your_profile_folder for details.
If you don't want to do that you can create an ICS calendar at a
location of your choosing. To do so, export your calendar as an .ics
file anywhere you like, then do File > Open > Calendar File and navigate
to the location. Once you have verified that events show up twice in
your calendar, you can delete the original storage calendar. Profile
backups just in case are suggested.
Philipp