How to boot a 2009 iMac from USB
OK, this is a very specific piece of information that hardly anyone will ever need… unless, like me, they’re trying to get an early 2009 iMac to boot from USB in order to install Linux.
Here’s what you need to know:
The 2009 iMac will not boot to USB using the Alt key with the Magic Keyboard that it came with.
However… if you have a USB keyboard around, plug it in to one of the ~32 USB ports in your iMac (srsly, this thing has a ridiculous number of USB ports). Then hold down the Alt key while booting. Choose EFI, then choose an option from the menu, and you’re in.
(Thanks to Old Toad in this discussion for this tip)