---
title: Mounting disk images on linux
summary: >
    I sometimes create full-disk-images of USB drives or hard drives and in
    rare cases I have to look into them. This is how I usually do that. (After
    I looked it up again ;-))
date: 2022-08-06T21:02:21+02:00
categories: [computerstuff]
tags: [linux,raspberry,reminders]

---

Let's assume you created a disk image with `dd` on a linux computer like

~~~console
$ sudo dd if=/dev/sda of=disk.img bs=4M status=progress conv=fsync
~~~

There are several partitions in that image and we want to access the linux
filesystem on it. For reference, I'll bring in some old backup I made from a
Raspberry Pi. That backup is taken from a 8GB sdcard, which is 2.6GB compressed
with `xz`.

When uncompressed, look at the partition table with _fdisk_:

~~~console
$ fdisk -l disk.img
Festplatte disk.img: 7,4 GiB, 7948206080 Bytes, 15523840 Sektoren
Einheiten: Sektoren von 1 * 512 = 512 Bytes
Sektorgröße (logisch/physikalisch): 512 Bytes / 512 Bytes
E/A-Größe (minimal/optimal): 512 Bytes / 512 Bytes
Festplattenbezeichnungstyp: dos
Festplattenbezeichner: 0x1b7f4bbb

Gerät      Boot Anfang     Ende Sektoren Größe Kn Typ
disk.img1         8192   532479   524288  256M  c W95 FAT32 (LBA)
disk.img2       532480 15523839 14991360  7,1G 83 Linux
~~~

We will refer to this output later again.

## Using losetup

The output of _fdisk_ is not that important to us, unless we have an unknown
disk image that we need to inspect first. I already know the partitions. The
first is the FAT32 partition used for UEFI and the second is the root file system.

### Creating and mounting the loop device

~~~console
$ sudo losetup --partscan --find --show disk.img
/dev/loop1
~~~

The second line is the output of the program. I used losetup already today, so
this is not _loop0_ but _loop1_. You may get `/dev/loop0` usually.

Mount the new virtual loop device to the directory that you like. This is
`~/tmp` in my case.

~~~console
$ sudo mount /dev/loop1p2 tmp
~~~

### Removing the loop device

~~~console
$ sudo umount tmp
$ sudo losetup -d /dev/loop1
~~~

## Using `fdisk` and `mount`

From the output above, we see that `532480` is the starting unit of the linux
filesystem in this image file. Further above you see the Units (Einheiten):
1 Unit is 1 sector of 512 Bytes.

I use a german speaking computer, so you might look for _Start_ or _Offset_ or
_Beginning_---you know what to look for...

We calculate the needed offset like: `532480 * 512 = 272629760`

And the resulting command is

~~~console
$ sudo mount -o loop,offset=272629760 disk.img tmp/
~~~

A remount is simple as

~~~console
$ sudo umount tmp
~~~

## When do you need this stuff

I often create quick and dirty (big) card images from my Raspberry Pies.
They are saved and easy to copy over to another storage (because they are
a single file).

If you have less space, `dd` is probably not the best method to create a disk
backup. `partimage` for example creates images from partitions, but it only
saves the used data from that partition. Those images are smaller.