I have the same CANON, and I use a class 4 on highest settings with no problem.
Mine also will not "let go" when I try to disconnect it from the computer. I contacted support and they told me that is not their problem.
It does the same thing on 7 computers and 4 Operating systems.
I also can not get it to download to comptuer at all with their software to which their response was it is not their problem.
Let me know if you have the same problems or if it is just my camera if you don't mind.
One note that I have found about HD recording is that you have to pay more attention that your subject is in focus, because it really shows. For christmas, I set mine up on a tripod and someone put a big present in front of the kids and the AF chose that to focus on, so the kids are very noticibly out of focus, but that present is crystal clear!