@amigadave, @Amigaharry2: thanks for your reply.

I have tried others ideas like:

- tring different memory configurations (512 MB to 3 GB with diffrent sticks): no result.
- booting the Mac OS X DVD (10.4): no result.

Here the screengrabs for Amigaharry2:

Printenv
http://obligement.free.fr/temp/printenv_g5.jpg

Devalias
http://obligement.free.fr/temp/devalias_g5.jpg

layout DH0 (Boot + Mac OS X)
http://obligement.free.fr/temp/dh0_boot_macosx_g5.png

layout DH1 (MorphOS partitions (the Boot partition is here desactived)
http://obligement.free.fr/temp/dh1_morphos_g5.png