Q: Is there are comprehensive method to consolidate my iTunes library?

The "Consolidate Library" command in iTunes is used to consolidate songs files stored at different locations into the designated iTunes Media folder (the one shown in iTunes preferences Advanced pane).  But those songs need to be in your ONE iTunes library already.  You cannot use any command in iTunes to consolidate songs that are in two difference iTunes libraries, into one iTunes library.

I've discovered that I've got a portion of an old iTunes library on a separate drive.

Is one these two iTunes libraries your current (most complete) iTunes library?  If so, for this "primary" iTunes library, are all of its song files organized by iTunes into one iTunes Media folder, designated in iTunes preferences Advanced pane?

Consolidation means that all media files in your iTunes library will be moved to the default iTunes Media folder, no matter it is on your hard drive or on an external drive. But it will not affect the media files in your second iTunes library.

You can import all your media files from the second library to your default library, but you will be losing play counts, ratings, etc.

Be sure to activate "Copy files to iTunes Media folder when adding to library" in the Advanced Preferences in iTunes before you do this.

First, the songs need to be in one iTunes library already.  Song files that are just stored on an external drive (and not in your library) are not part of the consolidation.  Songs in your one iTunes library, depending on the settings used when you added the songs (to the library), can be stored at various locations.  The "consolidate library" command

menu bar -> File -> Library -> Organize Library -> Consolidate files (checkbox)

consolidates and organizes your iTunes media files into the designated iTunes Media folder.  The location of that iTunes Media folder is set in iTunes preferences Advanced pane, where it says iTunes Media folder location (I have my iTunes Media folder on an external drive)  The default location for the iTunes Media folder (if you never changed it) is in your user account Music -> iTunes folder

<startup disk>/Users/[user name]/Music/iTunes/iTunes Media/

This action is a copy, not a move, so your original files (if they were stored outside of the iTunes Media folder) are left in place.  After confirming iTunes is now accessing its files from the iTunes Media folder, you'll have to do some "housekeeping" to manually delete the media files at the old locations to free up that space. 

If that different external drive has song files that are not in your current iTunes library, there is no simple automated way to consolidate.  You have to manually add those songs to your current "primary" iTunes library.  You have to deal with duplicates "up front" to avoid duplicates (as much as possible), or "after adding" to your iTunes library.