raspberrypi/documentation — pull request #111 by vk5tu (2014-08-11 11:41:35+00:00)
Title: Clearing partition table allows writing to a used SD card.

A "dd" on MacOS can fail due to lack of write permissions on the disk device, even when using sudo. This is a measure to protect SD cards from accidental overwritting. Clear the partition table on the SD card. This also sets the disk device permissions to allow writes.