Here's what I would try;
Choose the option to install BB onto C:\ drive while leaving the RealTracks on the external.

Afterwards 'copy' the RealTracks you want (by copying the associated folders) to the C:\BB\Realtracks folder, leaving the others on the external drive.

Then change where your BB/RB applications (on the C:\ drive) look for the RTs using Options or Prefs. With the above mentioned install method, they would be pointed to the external drive; change it to the C:\ drive locations for RTs and RDs.

Then you'll need to Rebuild your lists after launching BB/RB, so the program only sees and uses the RTs you copied over to your C:\ drive, yet you'll retain access when needed to the rest of the RTs. You should be able to copy more RTs over later if desired, and rebuild the lists.
Someone correct me if I'm wrong.

I do not work here, but the benefits are still awesome
Make your sound your own!