From the util-linux "mount" man page: "Note that, depending on the filesystem type, state and kernel behavior, the system may still write to the device. For ex

From the util-linux "mount" man page: "Note that, depending on the filesystem type, state and kernel behavior, the system may still write to the device. For example, ext3 and ext4 will replay the journal if the filesystem is dirty. To prevent this kind of write access, you may want to mount an ext3 or ext4 filesystem with the ro,noload mount options or set the block device itself to read-only mode, see the blockdev(8) command."