have a Hero 3 black edition, and had not used it until I took leave in December. I bought two Sandisc 64GB microSD cards, and after doing a short test with each one, used them to shoot video with my wife and daughter paragliding (a one off experience for them) and immediately afterwards, some amazing coastal rides (Chapman's Peak etc).
On the last of these rides we came across a motorcycle accident, and the footage might have helped, so I rushed to view it, only to find that both the cards came up with "the disc needs to be formatted before you can use it". No amount of coaxing, whether in the camera or a card reader, would let me in. I finally accepted that I would have to write off the footage, and formatted the cards, using the GoPro function to do it.

Bummer: both cards were still unreadable, and showing the error. I did some diligence on the web, and found that a number of cards, especially the large Sandisc, have a high failure rate.
Moral of the story: don't buy Sandisc, and start off with smaller cards (eg 4GB) and upload your footage often, until you are comfortable with it.