Sidenote to kaworu:
You need enough space to store the entire internal sd card (about 2GB) on the sd card plugged into the reader. If you do not have a second sd card, you can use netcat to send the image across a tcp/ip connection to your pc.
on you pc, do
netcat -l -p 8000 > sd.img
You can of course pick a port between 1024 and 65000 (yes, 65somethingthousand. Use a calculator).
On your kobo, do
dd bs=1024 if=/dev/mmblk0 of=- | busybox netcat <ip of your pc> 8000
I use this method because I do not have a spare sd card in my kobo.