The mounting issue is a result of one or more of the following:

• Problematic microSD card – card can’t be mounted. The card might not support SPI Mode.
• Incorrect format on microSD Card – card must be formatted FAT16, preferably with the 4D Systems RMPET tool, included with Workshop4.
• Insufficient current for the 5V supply – display might be flashing/resetting consistently.

Replacing the microSD card, reformatting the card, and/or reloading its contents, without reprogramming the PmmC/Application, would reveal if the card is at fault.

There is a test tool in Workshop4 called ‘uSD Tester’ found in the Tools menu. Put the card into the display, and load the uSD Tester application onto the display, and it will tell you if it can mount the card and potentially why.