How can I backup an MP3 player's firmware on Linux?

Hi, I want to install Rockbox on my sister’s mp3-player (a SanDisk Sansa e200) but I want to make absolutely sure I don’t ruin the device. I can copy all the files over to my harddrive but I also want to make a backup of the firmware if possible. I believe the dd command was able to do this, but since I’ve never used it and do not have any experience with it, I’m a bit cautious; therefore I’m asking this question ;-)

Thanks in advance.