There's an installer with any new RT's. Just follow the instructions and it will ask you where you want to install them. Point it to your HD using the drive letter assigned by Windows. Example, your system drive is usually "C" so if you had installed everything on your C drive, that's the default for any PG installers. Your external HD could be E, F, G, whatever. If you're not sure, go to "My Computer" with the HD plugged in and see what the drive letter is. What can mess you up later is if you happen to unplug your HD and plug it in again. Windows may give it a different drive letter so when you try to run Biab you get an error saying no RT's found. You then have to go into your RT preferences and change the drive letter.
This is one reason why Peter said earlier that even though everything will run fine from the external, if you have the HD space on your internal C drive, install everything there including all your RT's and use your external HD as a backup. If you do it that way like I do, then any updates or new RT's need to be installed on both your C drive and again on your external drive so both are current.

Bob

Biab/RB latest build, Win 11 Pro, Ryzen 5 5600 G, 512 Gig SSD, 16 Gigs Ram, Steinberg UR22 MkII, Roland Sonic Cell, Kurzweil PC3, Hammond SK1, Korg PA3XPro, Garritan JABB, Hypercanvas, Sampletank 3, more.