On a Mac PowerPC, if you happen to set the startup disk while using Mac OSX, you may find that the only way to boot into Linux after that is to hold down the option key during the boot process. This works but it takes a long time before the process even allows the choice of booting the Linux disk. I searched the Mate community as well as Google for a solution, but I found no direct reference to the problem. Finally, I discovered a simple solution in the Ubuntu wiki PowerPCFAQ page (https://wiki.ubuntu.com/PowerPCFAQ) although it took some digging to find it. I decided to write this in case anyone else using a Mac PowerPC experiences this problem and wants a way back to booting by default into Linux.
In the Ubuntu wiki PowerPCFAQ under 2. Installation there is a question , “8. What do I need for dual-booting?” Down in paragraph 7, I found the cause of my problem. If you use the startup disk control panel in the MacOSX preference pane you reset the PRAM and this prevents the yaboot boot loader from automatically starting. In that paragraph there is a link to the question, “I’ve lost yaboot, what can I do?” By the way, this is question 16 in the wiki under 7. Troubleshooting. In the fourth paragraph of that link is a link to “How do I configure yaboot.conf?” Without doing any editing of the file yaboot.conf and referring to paragraph 6 in that link, I used terminal to run: sudo ybin -v. After issuing that simple command, my system now boots by default into my Ubuntu Mate Linux system.