I've tested now some different boot-devices with MOS 3.12 and 3.11. I used plain installations from ISO. All devices have a small HFS-boot-partition (first on device) and a following, bigger and bootable SFS-partition for systeminstallation.

1. Booting boot.img and system from internal HD on IDE-port : MOS 3.12 is booting kernel, but nothing more - in early boot menue no IDE-devices visible any more.

2. Booting boot.img and system from internal CF-Card on 2nd.ide,device (CD1): MOS 3.12 loads kernel an boots completely to Ambients surface, but also no IDE-device recognized!

3. Booting boot.img and system from USB (ud): MOS 3.12 is loading kernel and boots only into a shell - loadwb fails(!!), but complete directories can be seen in shell. Seems there is also a kernel-problem accessing a USB-partition to boot MorphOS.
CF-partitions are seen and can be accessed, but no IDE-devices recognized.

Doing 1 - 3 with MOS 3.11 everything works as it should.
Loading MacOS X all partitions on HD can be seen.

I'm sure now, that this is a major bug in boot.img_3.12 in combination with these older PowerMACs!

If anyone can verify this behavior, please write it here. I will inform MOS-Team about that.