Your iMovie events and projects are contained in the Movies folder. Copy this folder to an external or network drive to back it up.
* iMovie will default to the library named iMovie Library
If you change the name of the library or are maintaining multiple libraries:
iMovie can manage libraries from other locations on your hard drive or from an external drive, but not a network (e.g.. scratch or P) drive. Do not open any library files directly from a network drive as this could lead to data loss.