Please see the:

‘Signal Noise Issues With MMC & SD Memory Cards (& Clocked Devices In General)

page in the resources area of our web site for details of a common problem experienced when using MMC and SD memory cards. Some manufacturers demo / evaluation boards have been found to suffer from this issue.

Many of the PIC microcontrollers have silicon bug issues that are detailed in the Microchip errata sheets. If you are experiencing problems with a PIC microcontroller accessing cards check the errata sheet for the device you are using.