Multiple media libraries would be a big job.
What might work for you:
1)in the search as you type field put in your portable drive letter: eg f:
2)click the < button that appears, to add all those files to a playlist.
You will now have a playlist called "f: Filterlist". If you add more files to your ML later you would need to repeat the process.
Shuffle Play: Tuniac will build an invisible playlist of all items in the current playlist in a random order. You can skip backwards and forwards on this invisible shuffled playlist.
Random Play: One off action. It will cause a forwards skip aka next track. The next track will be the opposite of the current Shuffle Play.
-if Shuffle Play is off, Random Next will skip to a "random song"
-if shuffle Play is on, Random Next will not skip to a "random song" but instead skip to the next file in the playlist.
Shuffle Play is a typical random/shuffle mode. Tuniacs shuffle does not pick a random song at the end of each song but instead builds an entire playlist you can not see that it will play from. The invisible list gets rebuilt if you alter the playlist eg via searching. deleting or adding files. The purpose for this is if you dont change the playlist Tuniac can guarantee it will play every song exactly once. Eg no repeats and nothing missing.
Random Next is a quick one off action similar to clicking the normal "next track" button but does the opposite shuffle mode to what is selected. Useful if you are listening to an album in order but you grow tired of it and just want something else randomly to play. Or useful if you are in shuffle mode and it is playing a song on an album and you think "oh I much prefer the song on the album after this" and want to quickly jump to that instead.
PS. I never use the Random Next feature and I doubt anyone else does