Miles and Jrough,
Miles asked whether he could just use the finder to copy the events and projects to an external hard drive, and then delete the entire original library. Projects can only be copied from one iMovie library to another -- they cannot be copied directly to an external drive unless copied into a library on that drive. The drive must be formatted Mac OS Extended (Journaled). Reformatting the drive from some other format would erase all data on the drive, so must be backed up first.
From a test I did, I believe that you can copy (not move) from the Finder package contents of one library to the package contents of another library. (You get to the Package Contents by Control-clicking on the icon of the iMovie library in the Movies folder, and selecting Show Package Contents from the drop down menu.) However, you should run your own test first because I haven't thoroughly studied this. I would want to run some more tests. Below are the results that I got.
Doing a test, I was able to transfer Projects from the Package Contents of Test Library 1 to the Package Contents of a newly created blank Test Library 2. In a blank Test Library 1 a created two short junk Projects with the iMovie app with random media in them. I checked the Show Package Contents of Library 1 to make sure that the Projects were listed there. I left Test Library 2 blank with no Projects in it. Next I copied (not moved) the following folders from the Package Contents of Test Library 1 to the Package Contents of Test Library 2, where they replaced the identically named folders in Test Library 2 (the test did not work without doing that):
--Sync--
CurrentVersion.plist
Settings.plist
Current Version...Flexolibrary
Then I went back to the Test Library 1 Package Contents and Edit/Copied its two random Projects and Edit/Pasted them into the Package Contents of the blank Test Library 2. I then opened the iMovie app in Test Library 2, and sure enough the Projects from Test Library 1 had copied over. The Projects also remained in Test Library 1.
My tentative conclusion is that if you copy the above listed 4 folders from the package contents of one populated library and paste replace them in the package contents of a second blank library the Projects will copy over. I did not test what would happen if the second library already contained Projects before I copied over the above listed 4 files. I suspect that those existing Projects would become unreadable., or maybe the procedure would not work at all. I did not try it with Events, although I suspect that the results would be the same as with Projects.
Confirm my findings for yourself before trying it on your real libraries and projects. If you want to try the above procedure, do a test first with test libraries, and backup your original master libraries before trying it with them. If you have a large master library that you want to split up, I would think that you first would need to create several blank libraries and copy/replace the above listed 4 folders from the master library into each blank library. Then, hopefully, you could batch copy projects from the package contents of the master library into the package contents of the destination blank libraries.
The above is not the conventional way of transferring projects, but since Miles asked the question I thought I would see if it can be done. Be very careful when doing anything in the Show Package Contents folders and subfolders, because if you move, retitle, or modify anything there you run the risk of corrupting your library.
I hope the above helps.
-- Rich