Sorry for the late reply, but I finally had some time to experiment with the black plug.
I did use the Sheevaplug-installer to make it multi-boot. That works fine now. I can also boot from the SD-card I used before in the white plug.
Because I used the installer, I now have a kernel without SATA support.
On plugcomputer.org it says: " 2.6.34 now has official support for the e-sata sheevaplug."
So I booted from the SD-card installed that kernel from http://sheeva.with-linux.com/sheeva/README-2.6.34
Installation was without problems. Only thing is, it is still booting from the old kernel.
This is because my environment variables has to be changed (correct me if I'm wrong somewhere).
Problem is I don't know exactly what this variables have to be, because (if I'm correct) I used the Sheeva-installer I now have a UBIFS system.
I found some variables but they were for jffs2.
I have to say that I didn't change the bootloader yet. I'm still using the one that was installed with the Sheeva-installer.
But if I understand it right that is only a problem if I want to boot from eSATA.
So to go short: what variables do I need to boot the 2.6.34 kernel I installed with sheeva.with-linux.com?
Thanks for your help. It's very much appreciated!