Juheeeeeeeeeeeeeeee! Finally, it worked!
So I definitely know, COS EXTREMEMORY ain't good for the n800!
There are still some "problems":
While copying with gnu-tar I got somekind of an error at the end.
Tar "said" there were files from date 01.01.1970. I don't know if tar just stopped copying then, or if this was just a warning. Booting works, but perhaps some files are not copied, after all? ... I got not input/output or dmesg errors, that's the important part.
Second subject... I can't connect via usb and got the message "[..] unable to connect via usb[..] Memory cards in use: internal memory card". Somehow it's logical, cause my ext2 partition is used as rootfs by the system and can't be unmountend due to the usb connection process. But what's with my first fat32 partition on the card? It doesn't show up on my PC Only the 128mb nokia card in external slot can be accessed from PC.
Any ideas, how to solve this?