Ubuntu :: How To Disable Video Thumbnails

I have Ubuntu 9.10, and writing video files to a mounted disk is slow. The reason is every time the buffer is flushed to disk, Ubuntu starts making a new video thumbnail for the file.If I skip the first block that has the RIFF data, then Ubuntu doesn't realize it's a video, and the write speed is about doubled. Then I can write the RIFF data at the end of the transfer (in the appropriate place in the file).I have tried setting an exclusive flock on the file while writing; however, that does not prevent Ubuntu from reading the file while it's being written and generating thumbnails.