Just a small update so people who read this thread don't get confused:

The last user did the mistake that he wasn't "root" (=administrator). If you run in this problem, then just type "su" in the command line and try again.

Getting external firewire harddrives to work:
1 - open a terminal (e.g. konsole)
2 - become root (type command "su")
3 - Load firewire modules (type "modprobe ohci1394; modprobe raw1394")
4 - use gscanbus to see if it was detected (type "gscanbus" and look if the drive is there)
5 - mount the new drive (type "mkdir /mnt/sda1; mount /dev/sda1 /mnt/sda1". Try other numbers, if this doesn't work.)

Quick'n'dirty. Correct if there are mistakes (just of my head).