I'm having a hard time following some of your terminology (import ... to my portable drive; import ... of the playlist library). You import file references into MC (not a drive); I don't know what a "playlist library" is. MC maintains a Library which consists of file references, their metadata, and playlists referencing those files.
When you moved the files from one disk to another, the MC library now no longer has any connection to those files. So you have to use the technique mentioned in the Moving Files wiki entry to reconnect them. If you've restored your library from a backup library, then you should be able to just modify the Filename field in the database using the Update database mode of the Rename, Move & Copy tool. Then you're done.
If on the other had, you imported the media files themselves into MC, and ditched the old MC references, then your playlists inside of MC will now be useless, since MC will remove invalid entries from the playlists (which would be all files you've moved, and re-imported).