Make sure your microSD card supports the SPI interface, that is how 4D processors connect to the microSD card. There are some brands of cards, also fakes or duplicate cards, in the market that fail to work with SPI either at all, or reliably. These cards are usually duplicates of the popular microSD brands and do not specifically state compatibility with SPI.
SPI compatibility is critical for 4D Systems processors.
For best results, purchase your microSD card from 4D Systems, We sell cards that we know are reliable and work as they should, refer to the 4D Systems website.