we may want to backup our SD bootable OS which have installed many application software. We can make SD Bootable OS into a SD card image.here is guide for you.This guide is for Linux platform only.

• Any Linux environment,​ we want fdisk, dd, tar, commands
• An SD Card which must have installed a OS

$sudo umount /media/*
$sudo mount /dev/sdb1 /mnt    
$sudo tar -cp * |sudo gzip -9 > ~/bootfs.tar.gz
$sudo umount /mnt

 $sudo mount /dev/sdb2 /mnt
 $sudo tar -cp * |sudo gzip -9 > ~/rootfs.tar.gz

$ mkdir WORK_DIR
$cd $WORK_DIR  
//creating empty image , this space is 2.5G,you can change by yourself
$sudo dd if=/dev/zero of=disk.img count=5000000    
//associating /dev/loop0 with the image
$sudo losetup /dev/loop0 disk.img               
$sudo dd if=/dev/zero of=/dev/loop0 bs=1k count=1024   
$sudo dd if=u-boot-sunxi-with-spl.bin of=/dev/loop0 bs=1024 seek=8  //write uboot in image

$sudo fdisk /dev/loop0

Using the fdisk command to create 2 partitions on the SD Card, e.g.

• 1st partitions start from 2048 sectors, 64MB in size
• 2nd partitions just keep it all default

Show bellow

Device Boot            Start         End      Blocks   Id  System
/dev/loop0p1            2048      133119       65536   83  Linux
/dev/loop0p2          133120     4999999     2433440   83  Linu

Also we recommend you to look at Bootable_SD_card

$cd $WORK_DIR
associateing /dev/loop0 with image again 
$sudo losetup -d /dev/loop0 && sudo losetup /dev/loop0 disk.img 
//associateing  /dev/loop1 with the first partition of /dev/loop0,1048576=512*2048)  
$sudo losetup -o 1048576 /dev/loop1 /dev/loop0            
//format the first partition
$sudo mkfs.vfat /dev/loop1  
//format the second partitions
$sudo mkfs.ext4 /dev/loop2   
$sudo mount /dev/loop1 /mnt   
//decompressing bootfs.tar.gz to the first partition
$sudo tar -zxvf ~/bootfs.tar.gz -C /mnt  
$sudo umount /mnt
$sudo mount /dev/loop2 /mnt  
//decompressing rootfs.tar.gz to second partition
$sudo tar -zxvf ~/rootfs.tar.gz -C /mnt 
$sync && umount /mnt
//cancel associating
$losetup -d /dev/loop2  
//cancel associating
$losetup -d /dev/loop1   
//cancel associating
$losetup -d  /dev/loop0