Getting the images back on the card
You might wonder why the above procedure didn't just use the card as a filesystem. The answer is that, in general, it is better not to use flash filesystems very heavily, because flash devices have much shorter life expectancies than hard drive media under heavy load. Thus, the complicated edits and copies are done to disk images, then the disk images are copied back as a single write operation.
Copy the initrd image in to the third partition of the SD card:
$ dd if=initrd.local of=/dev/sdd3 bs=16k
Likewise, copy the filesystem image in to the fourth partition of the SD card.
$ dd if=rootfs.local of=/dev/sdd4 bs=16k
Do not yank the SD card out of the reader right away. Sometimes it can take a few extra seconds for the system to finish writing. Wait until any activity lights have stabilized, then wait a few seconds longer just to be sure.