OverlayFS on Mikrotik RBM33G

Hi everyone!
I can't mount external disk as root via overlayfs. I did everything from here.
My config:

config 'mount'
        option  target  '/overlay'
        option  uuid    '38424f05-e921-4311-9ac0-088800f57675'
        option  enabled '1'
        option  fstype  'ext4'

I mounted to '/', /tmp/root' too but got the same result

In dmesg the following messages:

[   11.230915] mount_root: loading kmods from internal overlay
[   12.917015] mount_root: failed to mount -t jffs2 /dev/mtdblock9 /tmp/overlay: Invalid argument
[   14.074363] mount_root: switching to jffs2 overlay
[   14.079255] mount_root: switching to jffs2 failed - fallback to ramoverlay