Why is my Raspberry Pi file system read only?

Question

Why is my Raspberry Pi FS read only when I didn't make it that way?

Here is my error:

sudo apt --fix-broken install 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Correcting dependencies... Done
The following additional packages will be installed:
  raspberrypi-bootloader
The following packages will be upgraded:
  raspberrypi-bootloader
1 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.
4 not fully installed or removed.
Need to get 0 B/4,518 kB of archives.
After this operation, 57.3 kB of additional disk space will be used.
Do you want to continue? [Y/n] y
Reading changelogs... Done
(Reading database ... 191760 files and directories currently installed.)
Preparing to unpack .../raspberrypi-bootloader_1%3a1.20210805-1_armhf.deb ...
Adding 'diversion of /boot/start.elf to /usr/share/rpikernelhack/start.elf by rpikernelhack'
dpkg-divert: error: error checking '/boot/start.elf': Read-only file system
dpkg: error processing archive /var/cache/apt/archives/raspberrypi-bootloader_1%3a1.20210805-1_armhf.deb (--unpack):
 new raspberrypi-bootloader package pre-installation script subprocess returned error exit status 2
Errors were encountered while processing:
 /var/cache/apt/archives/raspberrypi-bootloader_1%3a1.20210805-1_armhf.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)

mount -l -t vfat; mount -l -t ext4 returns:

/dev/mmcblk0p1 on /boot type vfat (ro,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=ascii,shortname=mixed,errors=remount-ro) [boot]
/dev/mmcblk0p2 on / type ext4 (rw,noatime) [rootfs]

However, I can install and uninstall things fine. What's wrong?

Answer 1

If you "can install and uninstall things" the file system cannot be RO.

It is possible the boot partition is mounted RO.

run mount -l -t vfat; mount -l -t ext4 and include in your question.

It is likely your FS is broken; this is not unusual with a failed install.

I would just restore from my backup and repeat the update/upgrade.

Answer 2

The answer is in the output of mount: errors=remount-ro. If there are errors on /dev/mmcblk0p1, the /boot becomes read-only, and only that part.

So, what to do next?

fsck.vfat might be able to correct the partition, after which it would be possible to remount it rw. Unmount /boot, do the fsck.vfat and mount again under /boot. In practice, that would be:

sudo umount /boot
sudo fsck -y /dev/mmcblk0p1
sudo mount /dev/mmcblk0p1 /boot

If you have backed-up /boot with dd, you might dd the boot partition back. If you just backed-up the files, you will need to re-create the file system on /boot.

If you don't know why/how the file system got broken, I would pay some extra attention to back-ups. A failing sd card can also cause a corruption of the file system.

If your Pi does not contain much data or configuration (or if it is easily backed-up), you can try to re-install the Pi from scratch.