Grub menu gone!

[Jason]

Perhaps I wasn't clear, but both distros on the USB sticks booted successfully. However, normally, at the beginning of the boot sequence you get a menu with choices, and I didn't get this with either.

Firstly, I will admit that I'm just shooting in the dark here not having any experience with Apple devices but I'll try some other suggestion of what the problem might be or how to fix it.

If the grub menu is invisible that suggests to me that the problem is related to video, not grub. When the grub menu comes up, the bootloader is using a different video output, one that doesn't rely on a video driver. Or... you were on legacy mode before and now you're on EFI or vice-versa? I could see a system updating changing something like that. I'm thinking that maybe the reason you think Grub is invisible is that Grub isn't even involved. Apple is using its own bootloader and booting whatever it sees on the primary boot drive, however it sees it. Maybe that info might help. I don't know. I noticed in this tutorial for installing Linux on a Mac that they mention using UnetBootin to install it and create a boot chain that the Mac needs. This chain may have been disrupted. But how to repair it? Your guess is good as mine.

The other thing that just occurred to me is that maybe the reason you don't see grub is because the pause is set to 0 (if that's even possible) so it just boots the default? Check your customizer to see how many seconds it is set to. Maybe it's set to 0 on the disk and your grub customizer isn't changing it unless you explicitly say so.

[fox]

Shooting in the dark or otherwise, I really appreciate any suggestions. Maybe you'll find something I haven't. Attached are my grub settings, and as you can see, the timeout is set for 10, not 0. Maybe something else is awry in the /etc/default/grub file? Let me know.

I think your "different video" explanation is plausible. Referring to the tutorial you noted, I get the Mac Boot Manager when I press the option key at startup. This puts icons on the screen representing your boot options, and it always defaults to MacOSX. The Linux icon title is EFI boot. Selecting that doesn't bring up a grub menu nor grub options; it just boots whatever is the default set in grub. But when I start up the iMac without the option key, it is booting the grub default, not MacOS (unless I do a system upgrade on the Mac side, in which case it boots the Mac until you go to the options menu). The tutorial notes the use of Refind to get a menu at bootup that allows you to see icons for all boot options (options include anything bootable, including distros on the grub menu). This is what I was using until it stopping working. You have to reinstall Refind after any MacOS system update, because that update disables Refind. You install it on the MacOS side. I did this after the system upgrade as I always do, but it either no longer works or no longer shows. All of the above points to some display problem at bootup, which is hiding either the grub or the Refind menu. But I have no idea how to fix this. But on top of this, there is also the problem of Thunderbolt drives no longer appearing on the Linux side. I don't know how to fit that into the video explanation.