See Make a split library portable - Apple Community for background.
In brief, get a new drive that is big enough to contain everything, and another for backup. Copy your existing iTunes library folder to the root of that drive as X:\iTunes where X is the drive letter, press and hold down shift when launching iTunes, use the choose library option and open X:\iTunes\iTunes Library.itl. Reset the media folder (Edit > Preferences > Advanced) if needed to X:\iTunes\iTunes Media. Now with all of your other external drives connected use File > Library > Organize Library > Consolidate Files to copy in any files that are currently referenced outside of the main iTunes Media folder.
If you have more than one drive that only connects to the library when using the same drive letter (e.g. you have content on two drives that normally show as, say, drive F:) then connect one, make sure it is using the correct letter, consolidate, then repeat with the other. Consolidating can only copy in tracks that can be located.
Once the process is complete see Backup your iTunes for Windows library with SyncToy - Apple Community for thoughts on backing up.
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