Please be aware that during a next upgrade in your system, Debian tries to install a new version of Grub. The installation of Grub-ieee1275 will lead to a corrupted boot partition. Immediately after installation make sure you have hfsprogs installed, if not:sudo apt install hfsprogs
Then repair the boot partition:sudo fsck.hfs /dev/sda2
See here.
If you don’t repair your boot partition, you risk that your MintPPC system won’t be able to boot anymore.
Update 21 June: Another upgrade installed a new /usr/lib/xorg/modules/libglamoregl.so messing up X. I had to rename it again into libglamoregl.so.bak.