I tried it and was unable to remount read-only or unmount any of my ext2
partitions, even after changing to single-user mode. I got "device is
busy" messages from umount, even on a partition which had only been
mounted read-only. Normally this only happens when a file is open for
writing. I tried again, booting with init=/bin/sh:
# mount -n /usr -o ro
# umunt -n /usr
umount: /usr: device is busy
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Trevor Johnson