Thanks to you all for clarifying the situation.

I've found that I can buy a second internal drive for my own long of tooth and short of memory laptop (a Dell Inspiron 8600), which slides in in place of the DVD player, and I'm planning to go down this route, because although it's more expensive than a USB drive it's going to be much tidier on a gig, and probably less vulnerable to being dropped off the music stand.

I've got my desktop PC's CD and DVD drives shared on my network, so installation of software won't be a problem in the absence of the laptop's DVD drive.

I'll remove BIAB from the original drive and install the software and all the RealTracks on the new drive.

Nonetheless, I think it would be a good idea if BIAB could be revised so that it can look in more than one directory for big files like RealTracks, as there may be users who need to split up libraries as they run out of disc space.