How to format SD card for SD boot

This pages provides information related to formatting and SD card for a two partition configuration.

Table of Contents


Introduction

While some Linux systems only require a single FAT-formatted boot partition (e.g. those that use a ramdisk), some systems require a second ext4 formatted partition to hold the root filesystem.   In this tutorial, the FAT partition will be referred to as the "boot" partition, which is used to hold the boot image (BOOT.BIN) and Linux image (image.ub or Image + .dtb).  The ext4 partition for the root filesystem will be referred to as "root". 

Creating Two Partitions

These steps assume you're using a blank, unformatted SD card with no existing partitions on it. We'll use the fdisk utility in Linux to create the new partitions. Fdisk is text based tool used to create partitions on a disk. You can also use gparted which is gui base partitioning tool.

  • First, unmount your disk and start fdisk with your device name. In my case my SD card is at /dev/sdc: 
$ sudo fdisk /dev/sdc


  • Type p, to see your current partition table.
  • If you have existing partitions, you can delete them by typing 'd'
fdisk - check partition
  • Make a new partition by typing 'n'.
  • Make it primary by selecting 'p',
  • use default partition number and first sector .
  • Set aside 1G for this partition by typing +1G (this value can differ based on your needs - the rest of the card will be used for the second partition)
fdisk - new partition

  • Set the bootable flag on this partition by typing 'a'.
  • Now if you print out the partition table with 'p', you should see your new partition with a * under Boot.
fdisk - Bootable flag setting

  • Make the root partition by typing 'n'.
  • Select primary partition
  • Leave the first and last sector default.
fdisk - new partition

  • If you check your partition table now, you should see the 2 partitions you just created.
  • After verification type 'w' to write to disk and exit.
fdisk - print partition table

Formatting the Partitions

After creating the partitions, the next step is to format them with the appropriate files system. 

Boot Partition

The boot partition should always be formatted as FAT.  To format the first partition as FAT, use the following command:

$ mkfs.vfat -F 32 -n boot /dev/sdc1 

Root Partition

The root partition can be formatted as ext2/ext4 depending on your Linux configuration. To format the second partition as ext4, use the following command: 

$ mkfs.ext4 -L root /dev/sdc2 

If your root file system build artifact is an uncompressed ext4 image (i.e. rootfs.ext4) you can skip this step

Coping the Images to the New Partitions

Mount the partitions so you can copy your boot images and root file system to the card. 

Boot Partition

For the boot images, simply copy the files to the FAT partition.  This typically will include BOOT.BIN, image.ub, and boot.scr (2020.1 and later). However, your system may require additional files on the FAT file system. 

Root Partition

For the root file system, the process will depend on the format of your root file system image. 


roofts.ext4 -  This is an uncompressed ext4 file system image. To copy the contents to the root partition, you can use the following command: 
dd if=rootfs.ext4 of=/dev/sdc2

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