How do I create the iMovie 10 library on a different HD?

Agreed with your assessment that it's accomplished with hard links. As for it potentially little dangerous, I suppose it's only so in hands of the adventurous users who try to goof around and executing some ill-advised sequence of commands in Terminal. The typical users are safe although the strange appearance of hdd holding double copies of iMovie files that should overflow the drive is definitely confusing to anyone who is even just slightly observant.

At any rate, I fall into the typical user who doesn't goof around "under the OS X hood" (even though I could with my professional training). I just made sure I have additional backup just in case and followed what Apple says in this article (http://support.apple.com/kb/HT5917) to clean up the old iMovie folders and files after updating all to new library. All went well and my drive did get a bit of space freed up (about 20GB with a 570GB library). It wasn't the little bit of space I was after, I am glad to clean out the old library files so my on-going backup can stay clean too :)

You didn't read my post, one above yours.

SURE ENOUGH! There is a way!

Just as with iPhoto, hold the OPTION key when you click to launch iMovie! Keep it held down until iMovie presents you with a dialogue box, asking which project you want to open, OR if you with to create a NEW project, on ANY drive of your choice.

THANK YOU Apple for being consistent across various Apps!

Thanks Michael,

I was so confused using imovie for the first time that I did not notice this trick! I have now moved my Library to an external disk by creating a new one and copying contents from imovie and you can even choose which library you want to open by holding "option" whilst opening imovie.

Just hold down the Option key as you launch iMovie 10. A file system box will appear, asking you where you want to create the new proejct. Just like with iPhoto.

Should I manually move the Sharing, and Camera Archives folders as well? Also, if you move Theater, iMovie just makes a new one on the boot drive. Any way to fix this, or is that unnecessary because they should all be in iCloud?

Further to my post immediately preceding this one, I just discovered iCloud only permits uploads up to 15 minutes.