Apple is selling two different models of their Dual G5 XServe: One 'Normal' model, and another 'ClusterNode' Model. They are pretty much the same, but the ClusterNode doesn't have things you usually don't need in a rack-mounted 1U server anyway: CD-ROM and VGA-Card. However, it is also limited to a single hard drive.
I guess Apple's reason is that in a scientific cluster computing environment, the node's local storage is insignificant - whereas on a real server you most likely want multiple (mirrored) drives.
However, the significant price difference (Dual G5 ClusterNode has the same price as the Single G5 XServe) made me ponder buying a ClusterNode and adding another drive.
Fortunately, the hardware is quite similar. It turns out that the Mainboard has three SATA connectors, and the space for the 2nd and 3rd IDE drive was left empty. Also, the Backplane for Apples hotplug drives is not fully assembled - it is missing the connectors for the 2nd and 3rd drive :(
So Putting the drive in place and attaching it via a fixed cable to the SATA connector is no problem at all. However, Power is a slight problem. The whole machine has not a single standard power connector, so my only remaining option was to solder some wires onto the drive backplane PCB. This is ugly, but well.. who cares ;)
I'll put some photos of the modification online soon.