This procedure from (on May 16, 2022):

https://www.linux4sam.org/bin/view/Linux4SAM/SDCardBootNotice


For SD card boot, there are several files in the first partition:

BOOT.BIN  (at91bootstrap)
u-boot.bin (U-Boot)
uImage (kernel)


To make the SD card as root on a Linux PC:

dd if=/dev/zero of=/dev/sdx bs=10M count=1

fdisk /dev/sdx

Command (m for help): p
Disk /dev/sdx: 2002 MB, 2002780160 bytes
11 heads, 10 sectors/track, 35560 cylinders
Units = cylinders of 110 * 512 = 56320 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

        Device Boot      Start         End      Blocks   Id  System

Command (m for help): n
Command action
   e   extended
   p   primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-35560, default 1):
Using default value 1
Last cylinder, +cylinders or +size{K,M,G} (1-35560, default 35560): +64M 

Command (m for help): t
Selected partition 1
Hex code (type L to list codes): 6
Changed system type of partition 1 to 6 (FAT16)