There are two main reasons to move your iMovie files to an external storage drive. One is that video files take up a LOT of space, and the non-destructive editing that iMovie does can also add to the file load on your main hard drive (or SSD, in the case of a MacBook Air). The other is organizational – you might want to put all your stuff in one, easily accessed place for later retrieval and further editing.
Lucky for you, then, iMovie makes this pretty easy. Here’s how.
First of all, don’t move iMovie projects or events around in the Finder. This will end up breaking all the links between the files, making any future access an utter, corrupted mess.
Instead, hook up your external hard drive and launch iMovie. Just to the right of the Event Library is a cute little hard drive icon. That will show you what drives your Events are located on. Now, just Command-Click and drag the files from that window over to the new drive, and iMovie will make sure all remains connected and editable at a later date. At least until they upgrade iMovie a few more times.
You can do the same thing with your Project Library. With the little hard drive icon still clicked, you should see the project library organized by drive as well. Command-click and drag the project from iMovie to the external drive, and you’ll be asked if you want to move associated Events or just the Project on its own. For this exercise, choose to move the Events and the Project together. iMovie will make it happen all by itself and now you have all your stuff in one place. Nice, right?