How to Back Up an SD Card (Full OS Image) to an IMG File Using Linux Command Line

If your OS is installed on an SD card (common with SBCs, embedded devices, and lab/test rigs), you should keep a full backup image so you can recover quickly if the card gets corrupted. This guide shows how to create a SD card backup as an .img (or compressed .img.gz) using only Linux command line tools—then how to restore it later.

What You’ll Get

By the end you’ll have:

• A complete SD card backup that includes all partitions (boot + root filesystem + anything else)
• A restore method to flash the image to an SD card so the system boots exactly as before (OS + apps + configuration)

Requirements

• A Linux PC (or laptop)
• SD card reader (USB or built-in)
• Enough free disk space:
  • Uncompressed image ≈ size of SD card (e.g., 64GB card → ~64GB image)
  • Compressed image is usually much smaller (depends on used space)
• Tools (usually preinstalled): lsblk, dd, gzip, sha256sum

Creating OS Backup from SD Card

Step 0: Safely Power Off and Remove the SD Card

1. Shut down the device cleanly (recommended):

   sudo shutdown -h now
   

2. Remove the SD card once the device is fully off.
3. Insert the SD card into your Linux PC.

Step 1: Identify the SD Card Device

Run:

lsblk -o NAME,SIZE,TYPE,MODEL,MOUNTPOINTS

Look for the disk matching your SD card size.

Common device names:

• /dev/sdX (USB SD card readers; e.g., /dev/sdb)
• /dev/mmcblkX (built-in SD readers; e.g., /dev/mmcblk0)

✅ You want the whole disk device (example: /dev/sdb), not a partition (example: /dev/sdb1).

⚠️ Important: Choosing the wrong device can overwrite your PC’s drive. Double-check size and model.

Step 2: Unmount Any Auto-Mounted Partitions

If Linux auto-mounted partitions (like /dev/sdb1, /dev/sdb2), unmount them.

Example for /dev/sdb:

sudo umount /dev/sdb* 2>/dev/null || true

Example for /dev/mmcblk0:

sudo umount /dev/mmcblk0p* 2>/dev/null || true

Step 3: Create the Backup Image

You have two options:

Option A (Recommended): Create a Compressed Image (.img.gz)

This saves space and is ideal for most backups.

sudo dd if=/dev/sdX bs=4M status=progress conv=fsync | gzip -1 > sdcard_backup.img.gz

Replace /dev/sdX with your SD card device (e.g., /dev/sdb or /dev/mmcblk0).

What this does

• Reads the entire SD card byte-for-byte (all partitions)
• Shows progress
• Compresses output to save storage

Option B: Create an Uncompressed Image (.img)

This uses more storage but can be faster to restore.

sudo dd if=/dev/sdX of=sdcard_backup.img bs=4M status=progress conv=fsync

Restore the Backup Image to an SD Card

When you need to recover, restore the image to an SD card.

⚠️ Warning: Restoring will overwrite the entire target SD card. Verify the device name carefully.

Restore a compressed image (.img.gz)

gzip -dc sdcard_backup.img.gz | sudo dd of=/dev/sdX bs=4M status=progress conv=fsync

Restore an uncompressed image (.img)

sudo dd if=sdcard_backup.img of=/dev/sdX bs=4M status=progress conv=fsync

(Optional) Refresh partition info:

sudo partprobe /dev/sdX || true

Eject the card safely.

sudo eject /dev/sdX

Then insert it into your device, and boot. It should come up exactly as it was.

FAQs and Common Gotchas

Can I back up an SD card while it’s still running in the device?

You can, but it’s not ideal because data may change during imaging. The safest approach is always: shutdown → remove card → image externally.

What if I restore to a bigger SD card?

It will work, but the extra space may remain unused until you expand partitions/filesystems.

What if I restore to a smaller SD card?

It usually won’t work. A full-disk image expects the target card to be the same size or larger.

Quick Command Summary

Replace /dev/sdX with your SD card device.

Find device:

lsblk -o NAME,SIZE,TYPE,MODEL,MOUNTPOINTS

Backup (compressed, recommended):

sudo dd if=/dev/sdX bs=4M status=progress conv=fsync | gzip -1 > sdcard_backup.img.gz

Restore (compressed):

gzip -dc sdcard_backup.img.gz | sudo dd of=/dev/sdX bs=4M status=progress conv=fsync
sync