I don't know if that query syntax is correct (Windows Search is so poorly documented), or if perhaps Windows search is getting stuck on one of the files (what happens in Explorer?), but this kind of thing works here and in general:
Find IN "C:\Folder A" "C:\Folder B" "C:\Folder C" QUERY filename:*.png
I think I got it now. I suspect it wasn't ending because I was concurrently running a Find search. Also the quotes around "tag:X" should not be used. Thanks for the assistance.
Different disks and different folders are likely to be handled in exactly the same way. I don't think that would make a difference.
The query string being wrong is more likely. Looks like you came to the same conclusion. Windows Search seems quite finicky about quotes, and it's either inconsistent about how the work with different search terms, or I haven't been able to work out what its rules are. It's a shame Microsoft still haven't properly documented the syntax!