Can't Copy Large Files To External Drive

Thank-you both for your replies. The drive is formatted correctly, and running the repair feature in disk utility results in the message "No repairs were needed."

I have repaired permissions and run disk repair on the internal drive as well. Yes, I've played the clips in their entirety, but I'm going to do it again and look for any hiccups. That's an interesting theory and I want to check it out.
It has occurred to me that a workaround, although it by no means addresses the underlying problem, is to use QuickTime Pro to chop the clips up into several smaller files, copy them to the external drive, and then reassemble them via QuickTime Pro on the external drive.
FWIW, I neglected to mention (although I don't know if it's pertinent or not) that immediately after installing the external drive ten weeks ago I used SuperDuper to clone my entire internal drive onto the external firewire drive. I then erased the movies from the internal drive, which was bumping up against maximum capacity. I am able to boot from the external drive, and I want to retain this capacity should the internal drive (which is, after all, six and a half years old) fail.

FWIW, I downloaded the problem files again, and this time I was able to copy then all over to the external drive without a glitch. Perhaps the earlier files had been corrupt. In any case, I hope the problem doesn't reappear.
Many thanks to those of you who offered your assistance. It is much appreciated!