I also think that there should be an option to install at location of your choice - external or internal. No just supplied USB or C:\.
You can install BIAB on any drive you want. You can even install the program on one drive, the fonts on your boot drive, and the RealTracks and/or RealDrums on a third drive. My boot drive is drive C, the BIAB program is on drive F and my RealTracks are on drive G.
About the only place you shouldn’t install BIAB is to the /Program Files folder of your boot drive. That will cause headaches.