imovie, finished film, cannot edit

When you say export, was that Exported from the Share Menu? Like Share Menu>Export Movie?

Or did you transfer the Project/Events folders to the external hard drive by dragging them to the hard drive icons in the Event Library and the Project Library. To continue editing any iMovie project it needs to have both the Project Folder and the Events folders associated with the Project before iMovie will recognize it as a legitimate working Project inside the program.

Actually I've got some directions here you can follow:

Re-attach your hard drive to the Mac where you are currently editing the project.

Go back to iMovie then to View Menu > Group Events by Disk:

You should see your external hard drive in the Events Library. It will allow you to drag any Event onto that icon. Just Move the Events to the External Harddrive. Also, the hard drive will also appear in the list at the top, in the Project Library too. If you want to copy a Project to the hard drive, drag it from where it is now in the Project Library to the external hard drive icon. A dialog box will appear:

You will want to copy BOTH the Project and the Events to the hard drive. Once they are safely copied over, test them to make sure the open properly and you can see the video, and everything plays.

If the hard drive doesn't appear or has an yellow triangle with an exclamation point in the middle of it, you will have to erase and reformat the hard drive as a fully Mac compatible hard drive in order for iMovie to see it and use it.

I'm just happy if people can get their work done. It looks like you got this problem licked for now. Good Luck.