Hi,

I have a newbie type question.

I installed Ubuntu linux distro on an USB.

I have a Mac Book Air. I have seen the following, and don't quite understand the reason.

After having pressed "option" at boot time on my mac, I was presented with a disks from which to boot. I selected my USB stick. Thereafter every time I reboot my Mac, it will reboot from the USB if the USB is plugged in, or will enter a "grub shell" if the USB is not plugged in. To proceed, I enter "exit" and then my Mac is booted up.

The thing that I don't understand is how choosing to boot from the USB one time, has made a persistent change to the boot order. My expectation is that the Mac would always boot as default, and I would have to go to the boot menu via pressing the option key at boot up in order to boot from the USB.

Any help would be appreciated.

that is most probably set in the bios. Or you booted something from USB which installed something somewhere.... on the hard disk.

I would think a Mac/Apple forum would be a much better place to ask this question. The Apple support site at the link below explains how to change it back.

https://support.apple.com/en-us/HT202796