Hi,
Had a problem with an 8 min clip shot on a class 6 cheapie card (PRETEC) - It would not copy from either the camera, where it gave a warning that the camera had to be restarted on playback, or a USB reader where copying would result in a "invalid parameters" error.
I found that it was possible to use the edit function on camera to set an in point about 2 seconds into the clip, save the result back onto the card (once space had been made) as a new file - and hey presto it plays back on the camera and downloads without a problem.
This was a single clip of 30 shot, and it way in the upper 10% of capacity, so really i was asking for trouble.
If you find yourself in a similar predicament it may be worth a try.

Off to buy a few decent cards now!!