t's cracy, but I had now a similar problem on the exact same machine (MOS 3.17)! Suddenly, at next reboot, Quark-kernel could'nt find his MorphOS-Systempartition and hangs after loading kernel (blowers are going down, so kernel is loaded succesfull) without any message. Also Amiga-Amiga-CTRL will do reset ).
It depends not to the HD - first bootpartition (HFS) with bootimage is found (naturaly), and Mac-Partition with OSX loads normal too.
I've also a CF-Card with a 2nd, complete systeminstallation (bootpartition and systempartition MOS 3.15) at IDE-Bus, but the same problem. Changed to new HD to be sure , but does'nt have any effect.
Both, CF and HD, I can see dir of boot-partion in OF.

I tried also booting from CD -> same issue! Kernel hangs after booting.
It looks like that G5 does'nt "allow" to boot MOS-System from any media! So I'd got a hunch that this issue could'nt be located to HD, CF or CD.
Then tried to boot with edebugflags=init and it worked(!!) for some reboots. But some reboots later the same problem occured again. Removed HD and booting from CF (only with edebugflags=init) works again. Tested HD in other G5 - is OK and boots there complete!

And then I reset PRAM (Option+P+R) and all works fine again!!!
I'm not sure, whether this is the same issue to psergius problem, but, maybe, that will help too.

Crazy - what a chance that this happens right now to this thread. I never had such a behaviour on all my Macs since all the years I'm useing MorphOS.......