It seems to work for your test case, my test case (where you dd the image then create a partition), and even for the case where you create a file called casper-rw containing an ext4 filesystem in a vfat partition.
I've made a new initrd with this change in at https://people.canonical.com/~mwh/initrd.new although it's 60 odd megs so it might be easier to follow my instructions above to repack the initrd for yourself.
OK, that wasn't so bad: https:/ /code.launchpad .net/~mwhudson/ ubuntu/ +source/ casper/ +git/casper/ +merge/ 370351
It seems to work for your test case, my test case (where you dd the image then create a partition), and even for the case where you create a file called casper-rw containing an ext4 filesystem in a vfat partition.
I've made a new initrd with this change in at https:/ /people. canonical. com/~mwh/ initrd. new although it's 60 odd megs so it might be easier to follow my instructions above to repack the initrd for yourself.