SPI clock up to 84 MHz
Max SPI speed for Due is 42 MHz even if you set the clock divider higher.
The SAM3X/A embeds high speed pads able to handle up to 65 MHz for HSMCI and SPI clock lines
So the SPI library has this code.
if (clock < (F_CPU / 255)) {
div = 255;
} else if (clock >= (F_CPU / 2)) {
div = 2;
} else {
div = (F_CPU / (clock + 1)) + 1;
}
On Due you will need to slow SdFat, older SD cards only support 25 MHz max.
I use a clock divider of 2 for newer versions SdFat for a SPI speed of 42 MHz on Due.
SD.h uses a clock divider of 21 for a speed of 4 MHz on Due.
Start with a clock divider of 21 with the 1 GB card and increase it until it fails. SPI_HALF_SPEED has the value 4.