ext3

single user mode
# init 1
#mount -o ro,remount /dev/sda1
# touch /test
touch: creating `/test': Read-only file system
# mount
/dev/sda1 on / type ext3 (rw)

or
booting to rescue cd
unmounting sda1

Remove the journal feature from the file system (downgrade to ext2)

tune2fs -O ^has_journal /dev/sda1

fsck to delete the journal:

e2fsck -f /dev/sda1

recreate the journal (change back to ext3)

tune2fs -j /dev/sda1

and finally, remount it. On a live system, just reboot it.

Another option would be to change the partition to ext3 in /etc/fstab.