I agree that it would be good to have all your music on the external drive, and copies on your internal drive of those files you want to carry with you. So much easier and safer for backups.
I thought about this quite a bit, and every potential solution had a downside, or a failure mode. So I think I would have music both on the laptop and the external drive, and put up with duplicates.
What you are really trying to do seems to be;
Assuming: Your external drive is drive M:\ and your internal laptop drive is C:\.
Rule: If external drive M:\ is attached, show me only music files from M:\. If external drive is not attached, show me only music files from C:\
I couldn't work out how to do that globally, particularly as you would want to protect the record of files on M:\ even when the drive isn't attached.
However, there is a rule available in Views, under the "Set rules for file display...", that allows Duplicates to be removed. Now I assume that this just means that the records of the duplicates are removed from the view, and not removed from the library. What I don't know is how this rule would handle the situation where there are duplicates in the library, but one of the files isn't available because the hard disk isn't connected, while the other is available on the internal drive. If the rule automatically keeps the record for the files that is available, and discards (hides) the records for the duplicate that isn't available, then you have a solution.
If the rules works as I hope it does, just add the "Modify Results" rule "Remove duplicates of: Album; Artist; Name" and when the external drive isn't attached MC would show the local copy of the duplicated file, and when the external drive is attached, MC would show only one copy of the duplicated file. Which copy is shown wouldn't matter really, as long as it was identical.
WARNING: If you try using the above rule, back up your MC library manually first, and make sure it is a good copy. If the rules deletes duplicates rather than hiding them, you will want to be able to restore your library!
Also, if you can figure out a rule that only displays files that exist, then you would have a complete answer, using one library with all media recorded in it, display no duplicates, and only display files when they are available. I think I have seen someone describe such a rule previously. It may have been using the "Limit (Advanced)" rule.
EDIT: I see Brian has provided the rule for excluding missing files from the view.