It looks like you may be using the feature in iMovie 10 (File/Update Projects and Events) to update the projects and events into iMovie from an earlier version of iMovie, either iMovie 7, 8, or 9. Correct? Or you are just trying to open the projects in the earlier version of iMovie?
If the former, take a look at this thread and see if anything in there helps you:
https://discussions.apple.com/thread/250143128?answerId=250272113022#250272113022
You must copy over to the external drive the two folders in your Movie folder that are titled iMovie Projects and iMovie Events. Both folders need to be stored at the root level of the external drive, not in any other folders, so that iMovie can find them.
If you are just trying to open the projects in the earlier version of iMovie, copy the earlier iMovie app itself to the external drive along with the two folders that I described above.
-- Rich