Q: Importing Video on External Hard Drive

Hello Icebear!,

Thank you for using Apple Support Communities. 

I see that you want to access video files on an external HD and not have them transfer to the HD on your MB Air. You will have to import the video into your iMovie library to work with it, but what you can do is move your iMovie library to an external HD. Here are a couple of articles on how to do that, depending on the version of iMovie you are using. 

Move your iMovie for Mac (2014) library

iMovie ‘11: Copy or move a project to an external hard disk

and for a different perspective, here is another thread from the Communities that answers a similar question:

How do I store video clips on an external hard drive, using iMovie 10.0?

Have a great day.