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.
Merge pull request #111 from vk5tu/patch-1

Clearing partition table allows writing to a used SD card.