I've noticed a few people have had issues accessing there SD cards on a macbook
well its very easy to fix.
shut down the macbook and press left side shift, control, option button and power button all at the same time once. Then restart and once booted up you may insert SD card and it should work fine. This is is called resetting the SMC
more info here Intel-based Macs: Resetting the System Management Controller (SMC)