Where does iMovie save files

A friend uses iMovie (sorry, not sure which version) and saves his files onto a large external hard drive. However, he keeps getting messages that the hard drive on the computer is full cose to capacity. Does iMovie place backups, or any other files, on the computer's hard disc without you realising it? Is so, where are these files and can they safely be deleted?

Any other ideas?

Thannks