just in addition...the only two ways I found around this are:
before song is removed from collection, right click -> show in finder -> delete and then song shows up as missing in mixvibes and then I hit backspace.
However, I don't always do this and there are a bunch of songs not in my collection but are still on my disk. A way around this is to create a backup (including media) on an external disk, which will contain only the songs in my collection and then replace the current songs on disk with the backup...though this is a scary option cause if something goes wrong and stuff gets lost.....
what are your thoughts guys