If you only created a new partition it will not be formatted automatically. You'll have to do that on your own. Gparted can do that. Just right-click the unformatted partition (or chose Partition->Format as from the menu), chose a file system (I'd recommend ext2/3/4) and click apply.

Trying different folders but without success. I even created ed folder in /tmp/ (looks as /tmp/ed/). Still giving error. One question - what is the argument of the command mount? I mean "-o" given in the advices in the tread. Is it letter "o"? Small or capital one? Or it is "zero"? Probably I am doing something wrong. I switched the OS. I have win xp and ubuntu 12 on this laptop. I have downloaded again the file debian-m5-v3e-Sci-2GB.img.ext2.lzma and un-lzma-ded it in the linux. Put it also in /home/user/MyDocs/tmp/
Renamed the img file to be shorter to ed1.img.ext2
went to the console of N900
and
sudo gainroot
then
mount -o loop /home/user/MyDocs/tmp/ed1.img.ext2 /tmp/ed/
it gives again error about the argument.
What I am doing wrong?

http://lmgtfy.com/?q=mount+linux

Try skipping -o loop or replacing it with -t ext2. At least that's what I did the last time, IIRC.

It did the job. I used the following:
mount -t ext2 /home/user/MyDocs/tmp/ed1.img.ext2 /tmp/ed
and it has been mounted
after that
mount /dev/mmcblk0p4 /mnt/debian
cp -a /tmp/ed/* /mnt/debian/
and all was copied to the mmcblk0p4
What next?
Do I need to remove the mounting of the img file?
I mean to do:
umount /home/user/MyDocs/tmp/ed1.img.ext2 /tmp/ed

And someone was explaining need to be edited .chroot somehow

Yes, unmount both mount points, then edit /home/user/.chroot.
There are comments in .chroot, they should explain what to do.

The unmount command takes only one argument, e.g.
umount /tmp/ed
umount /home/user/MyDocs/tmp/blabla.img.ext2

Sorry I can't give you more guidance right now as I don't have ED installed and my phone has a (temporarily) broken WiFi so I can't even install it.

Hmmmm. This
cp -a /tmp/ed/* /mnt/debian/
takes a bit longer then I expected
Since now hour and 10 min and the phone is still on 100% loaded
Is it normal that? The "cp" has to copy 2,1G data from the img file to the new mmcblk0p4 partition. How much time it has to do it?

Over an hour is not unusual.

You can check the progress by running df -h in another terminal window. As a user is fine. Check out the line referring to the destination of your cp command.

Somehow when I put the following
mount /dev/mmcblk0p4 /mnt/debian
the phone starts to load the cpu 75-100%
I attached the phone as USB storage to Ubuntu and formatted the mmcblk0p4 as ext2. It was ext3
rebooted and again
mount /dev/mmcblk0p4 /mnt/debian
seems it is a bit better.
after that again "cp"
and following your advice in new terminal I entered "df -h"
.... useful
Now the problem I see is that the "cp" is doing too slow the things. It was copied just 40-45 Mb in about 7-9 minutes. Wow! It is too slow.
Then I get another idea.
Is it possible to mount the img file in Ubuntu? Then to mount with BootMenu the whole phone part (all its pertitions) to Ubuntu and to to this "cp" probably quicker than now.

P.S. CP process started at 10:25. Now 11:18 the mmcblk0p4 is with 347,8M of data. To get all 2,1G I will need to wait almost 5 hours. Woww

That seems way too slow, I agree. Not sure what the problem is. BTW ext3 is a bit safer than ext2, but also a bit slower. Not that slow though.

Yes, you can do the whole process on a host PC. It might be faster. Use the same commands, only adapt the paths accordingly.

Edit: You may not need to copy full 2 GB. The image is 2 GB. Think of it as an external hard disk. It is not entirely full, you will only have to copy the data.

Could you help me find out why BackupMenu does not mount the partitions of the phone as r/w. They are just readable. Is there specific keys/commands to enter in case to make it?

Edit 1 (few hour later)
Finally I did it. Now there is 4th partition mmcblk0p4 (ext3 type) over 4G. All files from the image debian-m5-v3e-Sci-2GB.img.ext2 are copied. All made trough Ubuntu
... And after restart the phone is still working
But the partition mmcblk0p4 is not permanent mounted. After the reboot it is missing. Probably I need to add permanent mount.
Then will read further how to edit the .chroot and to bring the icons up in the menu.