Solution to Yellow Triangle with Black Exclamation Mark
After a long telephone call and a lot of thinking I have found one of the many solutions to the yellow triangle with a black exclamation mark problem. In my case I was using sound effects in iMovie. I exited the program for no specific reason and when I opened it again to my dismay was the "triangle". I right clicked it but found no info to the problem. I later found that when I went into iTunes to change the name to make it organised the clip does not then work in iMovie. So I changed it back to the original name and it works!
Hope this helps,
Josh - PixelRanger
iMovie '08