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.
From 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.
In layman's terms /dev/rdisk goes almost directly to disk and /dev/disk goes via a longer more expensive route
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