I find that most older cards work at 8 MHz and almost all older cards work at half speed, 4 MHz. Many newer cards fail at all speeds with resistor level shifters. These cards expect very fast SPI signal rise times. The SD spec requires a rise time of less than 10 ns for all clock speeds under 25 MHz. It is just not possible to get sharp edges at 8 MHz into the typical capacitance with resistor level shifters.