I was writing to an SD card this evening from a Mac Book Air internal card reader, and the performance reported was 1.4 MB/sec. I later read tested it on the Raspberry Pi and got 20 MB/sec. It is a class 6 card and neither of these figures sound realistic!
I am using
dd for my tests. Results of identical tests of both read and write on both computers:
Raspberry Pi: Read 20 MB/sec. Write 11 MB/sec...
Air: Read 3 MB/sec. Write 1.4 MB/sec
Using these commands:
sudo dd if=/dev/zero of=/dev/mmcblk0p3 bs=1M count=400
sudo dd of=/dev/null if=/dev/mmcblk0p3 bs=1M count=400
This indicates that the perceived anomaly was in fact lousy performance of the integrated Mac Book Air SD card reader! It makes for very slow image creation.