Here's a site with a nice explanation on how to deal with how to save the the files or projects in iMovie. You can move your existing files and save new ones to an external drive:
http://www.sugarloaftech.com/index.php/2009/06/26/relocating-imovie-footage-to-a n-external-hard-drive/
Since you didn't answer my question about the camera I decided to do some digging. It's possible that you are using a JVC camcorder that creates .MOI (Movie Information - apparently the time stamp and other metadata) and a .MOV or a .MOD or a .TOD (the actual movie video and sound - but not really compatible with any OS). Neither of the two files can be read natively on a Mac or PC.
Both Mac's and Windows must use programs to convert the camera's proprietary format into something they can work with. iMovie 09 can import the video, but if you want to just copy the .MOV or .MOD file to your Mac and convert it you can use MPEG Stream Clip:
http://www.squared5.com/svideo/mpeg-streamclip-mac.html
Since your questions do not have much to do with the MacOS, but rather video production I'd suggest you look at the forum for
iMovie:
http://discussions.apple.com/category.jspa?categoryID=141
or
Getting started with video production on a Mac:
http://discussions.apple.com/forum.jspa?forumID=1136
Have Fun!
