According to the rasbery pi documentation, You can load your OS to a flash card with either /dev/disk or /dev/rdisk.
rdisk stands for raw disk.
/dev/disk is a block level device, why would rdisk be 20 times faster?
Using Mac OSX
Note: In OS X each disk may have two path references in /dev: /dev/disk# is a buffered device, which means any data being sent undergoes extra processing. /dev/rdisk# is a raw path, which is much faster, and perfectly OK when using the dd program. On a Class 4 SD card the difference was around 20 times faster using the rdisk path.