The only problem is that it cannot be mounted on a Mac (10.6+MacFuse+ExtFS) or Linux (11.04) . The Linux tells in "dmesg | tail" that it is 'not journaled'. Is it required that ext3 is jounaled to mount on a Mac or Linux ?
I already resolved it. During reformat to ext3 I forgot to reboot the device before executing mkfs.ext3.
After a second attempt everyting worked fine and both Mac OS (+MacFUSE 2.1.9) and Linux accepted MMC2 as an external USB file system when connected to the N810.