Thanks I get it. Found in ARHD rom details that Mike was supporting sd card ext4 partitions, but that's not for a primary partition if I get it right. With this tool actually you can read ext4 partitions (also external disks) under windows 7. I've tested that tool myself. Just thinking on an ext4 partitioned sd card for performance reasons, but looks like that makes not a big difference.
It's absolutely not supported or expected to have an ext4 partition INSTEAD of fat32 on our devices.
Samsung is differently.. they also use a completely different mechanism for making the sdcard available to your os.