Note : if the SD card had been previously formatted with a valid file system, usually it is automatic mounted when you insert the SD card to Linux PC. Check is the SD card has been mounted using mount (usually /dev/sdb *). Umount the SD card using sudo umount /dev/sdb *
Program the bootloader into offset 1K (1024B) of the SD card
Make sure bootloader (uboot , for 3DS or BBG3) image is used. See "Prebuilt image for using uboot ".
# dd if=your_bootloader_binfile of=/dev/sdb bs =1K seek=1
In above example, you should use no-padding version bootloader since it will be programmed to 1KB offset ("bs =1K seek=1"). If you do want to use the normal version bootloader (with 1KB padding at the head of binfile ), you can do as below.
# dd if=/dev/sdb of=./mbrbackup bs =512 count=1 [To backup MBR]
# dd if=your_bootloader_binfile_withpadding of=/dev/sdb [To program bootloader into offset 0 on SD ]
# dd if=./mbrbackup of=/dev/sdb [To restore MBR]