How do I select boot device?

[rcx142]

I have a 2008 Mac Pro machine, and a bootable USB. I've tested the bootable USB on my Windows laptop and it boots, it's using FAT32.

I cant boot into the USB. I tried two things that the internet said to do:

Holding the C key while powering on. Nothing changes. Maybe I need to be using an Apple keyboard and not a generic USB keyboard?

Setting the default boot device in System Preferences. I did that and selected a USB DVD (before I created the bootable USB was trying with a bootable DVD). After that when the machine boots I just get an eternally changing apple icon, error icon as seen here:

 

[mfram]

You can select a boot device by pressing the Option key while booting. It will bring up a list of choices. But a Mac is not a PC. If it's a PC-bootable USB thing, then it likely won't boot on a Mac. Macs use a different boot environment.

 

[rcx142]

Thanks, I had to use Left-ALT because I don't have an Apple keyboard but that worked insofar as it let me boot back into the hard drive so at least the machine is back to how it was before.