So I have an old PowerBook G4 and it's hard drive is dead. I want to install MintPPC to an external hard drive connected by USB. Everything works fine until I get to installing Yaboot and it says that it failed. What's going wrong?

Thanks!

The installer is unable to install Yaboot on the USB disk as it probably misses the code of finding the openfirmware path to that machine. The only thing that I can advise you, is to install yaboot manually. This is not easy for noobs. More info here:

http://mac.linux.be/content/yaboot

I would use a Debian live CD and from the live environment try to install yaboot in the Apple bootstrap partition on the USB disk.
http://live.debian.net/archive/images/6 ... werpc/iso/

Booting such USB drives:
I don't know whether Apple will boot with holding option at boot. If that would not work, you would have to boot from open firmware:
http://mac.linux.be/content/booting-open-firmware

All in all, not an easy project for Linux noobs....