I am running iMovie 10.1.12 on Mojave and am not able to create your issue. Movies in folders on an external drive import just fine. So the problem possibly is related your your upgrades.
It might be a permissions issue. Do a Control-click/Get Info on the folder and see what permissions are assigned. You want it to be read and write. Also, on the drive icon itself do a control-click/Get Info and, when the information box appears, scroll down to the bottom and click the box that says ignore permissions on this volume. See if that cures it.
Also, sometimes import problems can be cured by deleting preferences. To do that open iMovie while holding down the option and command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if you can import from the folder on the external drive.
You could also try redownloading iMovie. Drag the iMovie app from the Applications folder into the trash, but don't empty the trash. Then sign in to the app store and open your account by clicking on your name at the bottom of the app store side bar. When your account opens you will see a list of your purchases, including iMovie. See if you can redownload it there. If not, move your old iMovie app out of the trash and replace it in the Applications folder. However, the download should be successful. See if you can import after redownloading.
As a workaround you might try dragging the folder to your computer drive and from there dragging the media into iMovie. Or maybe opening the folder on the external drive and dragging the media from there into iMovie.
-- Rich