Howard,
Ericjcabrera's suggestion certainly works but please be aware that if you have "Copy files to iTunes Music folder when adding to library" preference checked on the Mac Mini side (it is checked by default), all music files will be copied from your external drive onto the Mac Mini's internal drive, which may not be big enough if you have a lot of files.
Ericjcabrera's suggestion certainly works but please be aware that if you have "Copy files to iTunes Music folder when adding to library" preference checked on the Mac Mini side (it is checked by default), all music files will be copied from your external drive onto the Mac Mini's internal drive, which may not be big enough if you have a lot of files.