your idea worked - testdisk saw the N900.dd.img. however it saw the "ot a boot d", a "N900" as FAT32 and something with very long numbers and JFS extension (?) but it was too big (32 TB), so I don't know what it was...

anyway, when I browsed the "N900", it showed the Easy Debian I think - the one which was on the n900... it also has a MyDocs folder, which is empty...
I also can browse the "ot a boot d", but with the same results as before... and one interesting thing: after a deeper search testdisk said "The following partitions can't be recovered" and I saw the "ot a boot d", "N900" and the third big partition... and as before it also said that can't write because None was selected...

thats all... I think I will have to try Photorec on the device itself and try to save the files on the micro SD card...

You may want to run Photorec each time for just one file type, like .jpeg, to have more control of the process and to choose the most important stuff first.

hmm, yes, you are right, thank you!

Dear michaaa62,

thanks for the help. I got almost all my most important data back.

I got some further issues though - I tried to reflash my device, but only flashing with the "combined" image worked flawlessly. flashing the emmc with "vanilla" image did not worked, although I tried so-so many ways (ehh, almost every way found on the internet...). after flashing the rootfs is booted normally, but the filesystem returned to read-only mode... when I tried to fsck the /home/user/MyDocs partition it always failed - after running and fixing things for almost 6-8 minutes it froze... so finally I reflashed the rootfs, installed backupmenu (from microSD card) and run it. from backupmenu I run fsck and it worked - after reboot I got almost a 1000 fsck_number_.rec files and everything seems to be working fine now... (I just wanted not only you to know that - I found that some users fail to flash the emmc...).