The MC-uSdMLC/4GB is a 4GByte Class 4 Multiple Level Cell (MLC) Secure Digital (Sd) Flash card. If performance is critical, you may want to consider the Single Level Cell (SLC) memory cards store one bit of data in each memory cell to help reduce ‘Read Fatigue’ errors common in consumer grade Multiple Level Cell (MLC) flash cards, which often store several bits of data in each cell.