Then this is a side-effect of saving the playlist when anything is changed. The purpose is to be able to save as much of it as possible in case of a Hoster crash. As it is set-up now, if you have a crash even the ones you imported from a singer's disk are saved at least until they have been played through or you close Hoster normally.
I don't know if they can fix a way around this. The best I can say for now is don't forget to hook up the drive.
Maybe if you can recreate it or have any others that you use frequently you might want to make a copy of them and put the copies some where else. Then if you do it again, then you will have a copy to copy back to the correct folder.