.. _l5_tips_microsd_boot:

Boot from the microSD card
==========================

You can boot other Linux distributions, including another PureOS, from the microSD card.
For example, you can install `Mobian <https://mobian-project.org/>`_ onto the microSD card and choose when to boot into it.

.. warning::

   The overall experience of using the OS if it is booted from the microSD card will be noticeably slower than running from the internal storage.

How to boot
-----------

When powering on the device, hold the Volume-down button.

Installing the same image
-------------------------

If you put the same image onto the SD card as the internal eMMC storage, you will end up with multiple partitions with the same UUIDs.
This may cause unexpected behavior during mounting in the boot process.

You can fix this by updating the partitions on the microSD card:

#. Install ``wl-clipboard`` to copy values from commands.

   .. code-block:: bash

      sudo apt install wl-clipboard

#. Ensure the microSD card is **not** mounted.
#. Change the UUID of the **boot partition**.

   .. code-block:: bash

      sudo tune2fs -U random /dev/sda1

#. Copy the generated UUID to the clipboard.

   .. code-block:: bash

      sudo tune2fs -l /dev/sda1 | grep UUID | wl-copy

#. Temporarily store this value somewhere like the Text Editor app.
#. Change the UUID of the **root partition**.

   .. code-block:: bash

      sudo tune2fs -U random /dev/sda2

#. Copy the generated UUID to the clipboard.

   .. code-block:: bash

      sudo tune2fs -l /dev/sda2 | grep UUID | wl-copy

#. Temporarily store this value in the same place as before.
#. Mount the **root partition** (``/dev/sda2``) of the microSD card.

   .. code-block:: bash

      sudo mount /dev/sda2 /mnt/mysdcard

#. Edit (as root) ``/mnt/etc/fstab`` (i.e. the ``/etc/fstab`` file that is *on the microSD card*) and replace the old UUID values.
   Put the saved UUID values on the line where the corresponding partition is mounted.
   The line that is mounting ``/boot`` for the first partition gets the first UUID value, and the line that is mounting ``/`` (root) for the second partition gets the second UUID value.

   For example, let's say the first generated UUID for the **boot partition** is ``9be41999-a89e-4ba9-af43-6b49f92d9932`` and the second UUID for the **root partition** is ``2d31dea7-39ed-4924-93ad-06b21114dc1c``.
   The ``/mnt/etc/fstab`` file should look something like the following:

   .. code-block::

      UUID=2d31dea7-39ed-4924-93ad-06b21114dc1c / ext4 errors=remount-ro 0 1
      UUID=9be41999-a89e-4ba9-af43-6b49f92d9932 /boot ext2 errors=remount-ro 0 2