I think this is a Mountain Lion issue. You will probably have the Panasonic MTS viewer plug in for Quicktime installed which enables you to view MTS files in Quicktime player.
Within your root folder the "PRIVATE " folder is now a quicktime player icon and won't open.
The workaround to view the MTS files is to right click on the icon and select "show package contents" and then again on the next folder etc until you reach the "STREAM" folder. Copy this folder to the destination of your choice and you will have the files which can then be viewed in Quicktime Player. This saves you the time of having to go through the same process every time you want to view the files.