With the default ext4 filesystem, mounting 'ro' doesn't actually prevent writes to the disk. For that you need 'noload' too, to prevent journal replay. This can

With the default ext4 filesystem, mounting 'ro' doesn't actually prevent writes to the disk. For that you need 'noload' too, to prevent journal replay. This can result in a broken filesystem if it wasn't unmounted cleanly, although assuming it's 100% read-only it shouldn't do any permanent damage (perhaps enforce read-only access at the block device level to be sure).