When I was cloning SD cards on Mac OS X using `dd’, it takes ages to get things done. I was using the following command:
diskutil unmountDisk /dev/disk2 sudo dd bs=1m if=~/Downloads/2013-10-09.alice.img of=/dev/disk2
It takes much less time when using /dev/rdisk2 instead of /dev/disk2:
diskutil unmountDisk /dev/disk2 sudo dd bs=1m if=~/Downloads/2013-10-09.alice.img of=/dev/rdisk2
The reason is that rdisks are “raw” thus resulting in a higher R/W speed, according to `man hdiutil` :
/dev/rdisk nodes are character-special devices, but are “raw” in the BSD sense and force block-aligned I/O. They are closer to the physical disk than the buffer cache. /dev/disk nodes, on the other hand, are buffered block-special devices and are used primarily by the kernel’s filesystem code.