I've deleted all the downloaded RTs
Let's distinguish between the downloaded installer packages, and the installed RealTracks content.
Download installer packagesThe download installers are realtracks*.pkg files, about 15 or so of them, ranging from 5-40 GB each, that will normally be found in whatever location you chose as your "Download Location" in the Install Manager. The only reason you wouldn't find them there, is if you had the option to "automatically remove downloads once installed" checked. If you have this checked, you should uncheck it.
Installed RealTracks contentThese are files in your Band-in-a-Box folder (and RealTracks folder). There are several different types of files for Drums and RealTracks, but some of the main files include R*.st2 files in the Band-in-a-Box/Soloists folder, and files and folders in the Band-in-a-Box/RealTracks folder. The "RealTracks" folder can be quite large because it contains the audio data (.m4a or .aiff files). For this reason, there is a setting in Band-in-a-Box that allows you to store your RealTracks folder on a different hard drive (volume). In other words you are allowed to have your RealTracks folder located somewhere outside your Band-in-a-Box folder. If you are installing a Band-in-a-Box package with the Install Manager you can specify the location of your Band-in-a-Box folder and you can specify the location of your RealTracks folder.
Running Band-in-a-Box and choosing your RealTracks folderYou run Band-in-a-Box by double-clicking on the "Band-in-a-Box.app", located in your Band-in-a-Box folder. /Applications/Band-in-a-Box/Band-in-a-Box.app. Since the default location of your RealTracks folder is IN your Band-in-a-Box folder, that's where Band-in-a-Box will initially be looking for RealTracks. IF you have RealTracks folder in a different location, then you will need to specify that location in the RealTracks Preferences.
Moving Band-in-a-Box from one hard drive to anotherA complete Band-in-a-Box UltraPAK install is about 170 GB. If you currently have Band-in-a-Box on your internal hard drive and it's working fine but you want to move it to an external hard drive (which is plugged into the same computer), there is an easy way you can do this. Just create an /Applications folder on the external, and copy the Band-in-a-Box folder from the Applications folder on the internal to the newly created Applications folder on the external - just copy, paste, and wait.
So now that we're on the same page, for your specific situation, you need to:
1. Decide where you want Band-in-a-Box installed. In my opinion, you should put the entire Band-in-a-Box folder on one of your external drives, then there is no confusion about where the various parts are.
2. Now that you've decided that, you need to figure out where you stand. Do you have Band-in-a-Box installed ANYWHERE already? As a sanity check, select each of your Band-in-a-Box folders on your SSD, your 2 TB HDD, and your internal HD, and Get Info (Command+I) on each. What is the size of each? Any of them close to 170 GB, or in the ballpark?
Why don't you reply with the sizes and we'll go from there.