Locate the library on the external drive and rename the files:
iTunes Library.itl
iTunes Library.xml
iTunes Library Extras.itdb
iTunes Library Genius.itdb
to say:
iTunes Library (Old).itl
iTunes Library (Old).xml
iTunes Library Extras (Old).itdb
iTunes Library Genius (Old).itdb
Then use the option-start-iTunes method to create a new empty library somewhere convenient such as the desktop. Close iTunes and copy the files:
iTunes Library.itl
iTunes Library Extras.itdb
iTunes Library Genius.itdb
into the iTunes folder on the external drive. Option-start-iTunes once more to open this copy of the empty library, then use File > Library > Import Playlist to import iTunes Library (Old).xml.
tt2