My cards worked for about five weeks (of heavy shooting--we are a news station) and then started to crash. Record would stop in the middle of a clip and I'd get an invalid media error message. The clip that I was shooting would not be recoverable.
The problem happened repeatedly with both cards, which led me to think it was a camera problem, but when I got new cards (one the same model SanDisk and one Sony) the problem went away.
I looked in the Manual and these cards should work properly, as SanDisk class 10 cards fall within their recommendations, but obviously I've had problems with them.
I've looked around for any other info and haven't found any. I've also looked for new firmware but haven't seen any. I'm trying to get a hold of JVC support as well but I thought I'd post here in order to cast a wide net.