You can add several to the list. But if you add a path like /media/disk, you can't add /media/disk/recordings because it's already scanned and vice versa.

You should add as many paths as your can like /media/disk/music and /media/disk/recordings etc. so you can rescan just one selected path without having to go through all your media.

EDIT: Be sure to add your VDR channels to collection too with ./install/vdr. It should then automatically scan them and allow you to stream them if you use streamdev.