My Arduino Due finally arrived and I started testing SdFat.
The results are disappointing since the standard SPI library is slow. I used an SPI clock of 42 MHz.
When I run the SdFat bench example with large 4096 byte reads and writes, the results are just a little slower than the same test on an AVR Mega with an 8 MHz clock. The Mega is fast since I wrote optimized SPI functions for AVR. The maximum read/write rates are about 600 KB/sec for a high quality industrial SD.
I checked with a scope on the Due's SCK and the result is that a byte is transferred in about 200 ns then there is a gap of about 1250 ns until the next byte.
The Due SPI library needs optimized block read/write functions.
On Teensy 3.0 I get over three times the performance of Due. This is with an 24 MHz SPI clock on Teensy 3.0.
The maximum read/write rate is over 2 MB/sec on Teensy 3.0. Due should be faster with a 42 MHz clock.