No transfer errors, no write errors (probably the most important issue to me) and they are fast (I haven't been able to overrun the buffer yet, more than likely because of the card speed).

Confused, I checked the error and just saw "BUSY" with the write light on my camera just solid

It has caused no errors and lacks unlike an SD card in the second slot in 5D Mark iii.

When I go to download my videos to my laptop, this card always reads as an error and I lose my work.

6 out of the 20 cards failed and showed error corrupt card in the camera..