If I click, from the Start menu, my 'My Music', i have several folders there grouped by artist name.
If I right-click any of those artists, I can choose "Add to Now Playing List"
These are all the songs that I have on my primary partition.
I have a secondary partition, which is a volume mount point (labeled as Albums) where I have entire albums stored.
If I right click 'most' of the albums there, I get the same "Add to Now Playing List" option. But there are a couple that do not. My guess is that they aren't recognized by Window's as properly-creted Music folders (for whatever reason)>
So, where's that stored? The registry, presumably. But I've been unable to locate it...