How to Group Video by Events in iMovie
You can group all your videos in a single unified video library based on events. Although iMovie Library groups all events by a given year, you can sort them by hard disk instead (assuming that you have more than one, of course) by clicking the little button with a hard disk icon in the upper-right portion of Event Library.
Here are some of the options you have for organizing your events:
Merge them: You can take video from multiple sources and place them into one event. Merge Events by choosing File→Merge Events.
Split them: To split one event into two, click the clip you have in mind and choose File→Split Event Before Selected Clip.
Drag them: To move a clip from one Event to another, drag it to the title of the new event in the Event Library. Hold down Option while dragging, and you copy the clip rather than move it.
When you import video into iMovie, it’s organized into events. Make sure to provide reasonably descriptive names for these events: My Little Girl’s Ballet Recital, Thanksgiving Pig-Out, whatever. If you didn’t type a descriptor when you imported the video, iMovie will substitute one for you, something like New Event 8-13-08.