This is what I would try, here at Suffolk Community College we use JVC Everio hard drive cams with iMacs. But we capture to the internal drive and have no problem, how long are the clips the students are capturing? If they are large it seems like the external drives cannot keep up with writing the info. (HD has a lot of data.)
Transfer to the internal drive if you can, then copy to the external drive the .mov files that result. Or transfer just a few clips at a time to the external. We have 750gig hard drives on iMacs and have no problems.
Also we found that the students were using the battery power to transfer after shooting, the batteries were used up and transfering crashed. Hey you can contact me vie email if you want. GBracco@mac.com
Greg