Pay attention to the options during the install.
At a given point you are offered the option for a Minimal install.

It will show you that you can install only the program(s) to C: and let you 'install' everything else wherever you want. Including the existing USB hard drive you are installing from, in which case they won't even need to install as they already exist.

One caveat with this choice is that if you move that drive to a different USB jack or a Windows update changes things (etc), you may have to re-establish the location of the Realtracks/RealDrums in the program's Options-Prefs area so it knows where to find them, as MusicStudent referenced.

My laptop is set up like this ..


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