[MacDV] iMovieHD video freeze

[DGH]

I'm making an iMovie in version 5 which is HD. Movie is 48 min long.  
Working from external firewire drive.
Suddenly the video fails to move in the timeline. Sound is normal.  
Video stops at first frame of a clip as movie proceeds. Up to now all  
was normally proceeding as I added clips and voice.  I have made a  
lot of iMovies this way and this length.
17.1 GB available on the computer.
  202 GB available on external drive.
  Using Mac OS 10.4.11 Power PC  G4

Suggestions?

George