Copying an iMovie movie

I made an iMovie that is 3 hours long. It plays fine from the iMovie app however, when I "share" it with a USB stick it shows as complete and it shows as 3 hours on the stick but it only plays up to about 2 hours then video and audio freezes and the counter continues. I have made quite a few movies in the past with iMovie and never encounted this. Any suggestions?