You have not mentioned what card reader you are using. I use the Lexar card reader which handles both CF and SD. I do remember that I needed to update the firmware for the Lexar Professional 1000x UDMA 7. I have used this card since March 2012 with no problems. Do not underestimate the importance of a quality card reader with the proper firmware.
5DC, 5D3, Elan7, G12, 28 1.8, 50 1.8 II, 85 1.8 USM, 135 2.0 L, 24-70 2.8 L, 70-200 2.8 L, 560 EX, 580 EX II (2) --- all Canon.