Must a 20.04 live-boot installer be UEFI/GPT to create a UEFI/GPT SSD install?

[watchpocket]

This was run from the installer: https://paste.ubuntu.com/p/p8hDhtQytR/

[oldfred]

While you have booted in UEFI mode to run Boot-Repair and still have an old ubuntu entry in UEFI, your install is BIOS.
And you are using the old MBR(msdos) partitioning.
If you set system to boot in BIOS mode, does it not boot?

I do prefer gpt whether BIOS or UEFI. But you have to have extra supporting partitions.
If BIOS, you need bios_grub partition 1MB unformatted for BIOS boot
If UEFI, you need a FAT32 partition with boot,esp flags for UEFI boot.
When I still had BIOS only system, but planned on newer UEFI system, I put both as first two partitions on all new or reformatted drives, including larger flash drives. Now with UEFI, I only add the ESP.

[watchpocket]

Here is an info report with my two internal drives unplugged.

(Those internal drives are a BIOS 18.04 and a non-OS, non-booting SSD just for data storage. Those internal drives are what you saw in the boot-repair report above.)

This report is just of the UEFI live installer (sda) and the external SSD I'm trying to install the UEFI/GPT 20.04.1 on (sdb).
https://paste.ubuntu.com/p/HsJF6rSBK9/

[oldfred]

UUIDs look ok
The only issue may be that drive is seen as hd1 and USB installer is hd0. But when you reboot the boot drive will be hd0. But UUID should override that.
You could use Escape right after UEFI/BIOS screen to get to grub menu and edit the hd1 entry to hd0.
I often have to do that with my grub2 loopmount ISO boot as drives can change.

[watchpocket]

I'm taking a step back before I continue my UEFI/GPT setup to learn some fundamentals, starting with this, which is recommended for anyone with limited understanding (as mine is) about how it works. It's also a good read, and very clear.

[oldfred]

That is one of the many links I have to explain UEFI installs and details including many Acronyms and links to details on them.
See link below in my signature.

[watchpocket]

Finally I got a bootable UEFI/GPT Ubuntu-MATE 20.04 installed on a removable drive.

After installing the iso onto a target SSD (carefully UN-checking the option to load third-party apps, because the installer will crash toward the end of its run if you do) I at first followed the steps in Part E of this tutorial. But the first time I did it failed -- I didn't understand what a chroot jail was, nor why I was creating a chroot. I also followed the completely mistaken instruction in this guide where he says:

"First we’re going to unmount the media volume of the thumb drive (leaving ‘Try Now’ Ubuntu running in memory only)." Also, his use of non-specific terms like "in your system," when he should have specified that he was talking about the root partition of the target drive, didn't help. I actually tried to unmount the thumb drive "try it" system that I was booted from, and couldn't.

A better guide is here. The key for me was where he says: "installing the bootloader is only possible from inside the Linux installation we want to boot. However, we need the bootloader to boot that installation, leading to a Catch-22 issue. The solution is to run the bootloader installation through a chroot jail."

At that point I understood that I needed to UNmount the root partition of the target SSDso that I could re-mount it (and the ESP) in the chroot jail, after setting up a small working environment there.

You also have to put the right line in /etc/fstab, load the efivars, and install grub correctly with the --removable tag from inside the chroot /mnt dir -- it's all explained in the guide. It's all a bit tricky, you just have to pay attention and think about what you're doing. I didn't want to enter any commands until I completely understood every aspect of what a command (or set of commands) did and why I was using it.

Now, booting to the removable drive works, either with our without my internal BIOS-run Ubuntu-MATE 18.04 SSD connected. I do have to select "INTERNAL EFI" from the pre-grub menu, and I do first get an error message saying, "System Boot Order not found - initializing defaults," but it then does go ahead and boot to the removable SSD's 20.04 login page. (This box does not have Secure Boot.)

So I may still have a few kinks to work out, but I finally jumped through the hoops necessary to get a working bootable removable-drive system. Therefore, I'm now marking this thread solved. Thanks to all for the help and responses.