Comment 8 for bug 309762

Confirm this bug.
I've tried the break=mount option too without success. I can mount an ext3 volume but still not an ext4.

$ mount /dev/sda1 /root
results in (obvious as ext3 defaults):
EXT3-fs: sda1: couldn't mount because of unsupported optional features (40)
mounting /dev/sda1 on /root failed: Invalid argument

$ mount /dev/sda1 -t ext4 /root
results in (sda1 is ext4):
mounting /dev/sda1 on /root failed: No such device

In my case I made a conversion ext3 to ext4 using this guide
http://kernelnewbies.org/Ext4
that is,
$ tune2fs -O extents,uninit_bg,dir_index /dev/sda1
$ fsck -pf /dev/sda1

uname -r
2.6.28-3-generic

So I guess it's up to what ext4 features you activate that makes the ext4 device mountable or not in the initramfs shell. However, I can boot up another Jaunty system an mount this "sda1" device there with success so the device itself is ok.