On my Mac, can I simply create a new partition in Disk Utility, install an Operating System to it, such Arch Linux, and then boot from it by holding alt/option during boot (Startup Manager)?
2 Answers
I don't think so. On my Mac, even though I don't have OS X (I think its all the same though) you don't have to use option to boot into Ubuntu, but any other OS like OS X you will have to use Option to boot into it. EDIT : Ubuntu or anything like Linux WILL NOT show up in the Option menu. It just boots straight away to Ubuntu.

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Yes, except the last one:
and then boot from it by holding alt/option during boot
Only Mac operating systems and sometimes Windows will show up in the menu. Instead I would suggest installing rEFInd. I have used it to dual boot Kali Linux and macOS Sierra on my MacBook Pro.
Very simple to install and use except you will need to turn off SIP for the installation process. You can turn it back on after.
To Disable SIP:
- Reboot your Mac into Recovery Mode by restarting your computer and holding down Command+R until the Apple logo appears on your screen.
- Click Utilities > Terminal.
- In the Terminal window, type in
csrutil disable
and press Enter.- Restart your Mac.
Install rEFInd
Enable SIP
- Reboot your Mac into Recovery Mode by restarting your computer and holding down Command+R until the Apple logo appears on your screen.
- Click Utilities > Terminal.
- In the Terminal window, type in
csrutil enable
and press Enter.- Restart your Mac.

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EDIT: See http://www.rodsbooks.com/refind/sip.html
– BaconDuctTape Apr 23 '18 at 22:17