I had a similar issue on my g5/1,8GHz with MaxtorHD. This HD is too slow for MOS to be rcognized after booting kernel.
(remember: Kernel will be booted from boot-partition by open firmware - not MOS!)

If you boot via boot-loader-script, you may add the ED=init - command to slow down kernel a bit:

for example.: boot hd:,\boot.img ramdebug BD=SYS_OS ED=INIT
in that example boot-partition (HFS-filesystem) has to be first partition and name for MOS-bootpartition is SYS_OS: (or whatever you named it).
You can try booting by typing it directly in OpenFirmware-Console - if HD is recognized - voila!

If you boot by built in MAC-bootloader (tbxi-file) you may change following line in bootinfo.txt

boot &device;:&partition;,\boot.img ramdebug
to
boot &device;:&partition;,\boot.img ramdebug ed=init

Don't forget to "bless" that file after saving. (shell: HFSSetMacboot bootinfo.txt)


Or you can change HD.