grub cannot boot from ext4 on mmc ("unknown filesystem")

[Andreas Ufert]

Hi,
here's my setup:

grub 2.06
build with yocto/kirkstone

grub (arch i386-pc) is installed on (hd0) (MBR) and (hd0,msdos1) (modules 
and config)
kernel to bee booted resides on (hd0,msdos2)

grub fails with "error: unknown filesystem"

grub *can read* (hd0,msdos1) (vfat filesystem):

        ls (hd0,msdos1)/
        grub/
        grub>

grub *can see* (hd0,msdos2), but *cannot detect* a filesystem:

        ls (hd0,msdos2)
	Partition hd0,msdos2: No known filesystem detected - Partition start at 
33792KiB - Total size 2097152KiB

grub *cannot read* (hd0,msdos2)/:

        ls (hd0,msdos2)/
        error: unknown filesystem


All relevant modules are loaded, particularly ext2.


If I dd the very same image onto an USB key grub perfectly reads the ext4 
filesystem and boots the kernel so one could argue that MMC is the problem. 
However, the vfat partition can be read from MMC just fine. It's only the 
MMC/ext4 combination that fails somehow.


I've run out of ideas... Do you guys have any?


Thank you for your time

Regards, Andreas