I am using a MacBook Pro with 2G free.
Yes this is way to low.
I never let my MBP go less than 25Gb (when all material are imported) !
So You need to free up lot's of lot's of space to get iMovie to work as intended.
If You use an external hard disk to off load some old projects onto - THEN IT HAS TO BE - Mac OS Extended formatted - ELSE it will go terribly wrong as UNIX/DOS/FAT32/Mac OS Exchange - WILL NOT WORK.
Yours Bengt W