Is this possible? I can't get it to work.

Made a bootable USB stick with Rufus. I've done this before with other ISOs and booted UEFI just fine, recently, same machine, same USB stick. (New machine for LinHES or some other MythTV variant, whichever I get working.)

Should it work? Is it not supported? What do I lose, really, in legacy mode, if I'm only using the machine as an "appliance," not hot-plugging hardware, etc.?

I've tried googling "linhes uefi", "linhes efi", constrained search to site:linhes.org, searched directly on this forum, no results in the past 5 years, and those few I found were other people having problems but no solutions.

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Not using LinHES because 8.6 didn't support my hardware, but still very interested in future developments, so lurking once in a while.

I know this dos not answer most of your question but I used Rufus 3.5 on my last install (LinHES_R8.6.1.iso) and it seem to work fine. I had to fight with my bios to get it to recognize the USB drive. I have older hardware with DDR2 memory. You might have to sacrifice a DVD if all else fails.
http://linhes.org/projects/linhes/wiki/FAQ

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TVBox
LinHES R8.6.1

TVBox, it works fine in BIOS mode, but not UEFI.

The main problem with that is, the installer doesn't "see" the nvme SSD I want to install to, just the HDD.

So here's what I've done so far.

Unpacked the ISO. It doesn't have the files for efi.

Unpacked a full arch linux ISO. Combined and modified some files into the LinHES files and made a new ISO, burned to USB stick with Rufus as GPT, and booted off this.

Took several tries, but I was able to get linhes-installer to run and complete successfully.

However, it still didn't see my SSD, so I think I need to unpack the airootfs and see if that also needs modification.

I'm very surprised LinHES doesn't support this out of the box. UEFI is not new, and the main Arch install supports it.

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Not using LinHES because 8.6 didn't support my hardware, but still very interested in future developments, so lurking once in a while.

Here's my progress so far...

I unpacked the ISO. Seems like it doesn't even have the files for EFI booting.

Unpacked the current Arch ISO, and combined/modified files (including mounting and modifying efiboot.img, and some cfg files) then used mkisofs to make a new ISO.

Rufus was able to write this to a USB stick, and use GPT instead of MBR. It boots up in UEFI mode, and the installer runs just fine.

However, the installer (is this MythVantage?) doesn't see my SSD which is on /dev/nvme0 (if I remember correctly.) But installs on the HDD just fine.

I have to manually select the boot device as the HDD after installation, and it boots fine.

If I open a shell and type 'parted -l' it lists my SSD, which has a previously installed OS on it (xubuntu) so I see nvme0p2, nvme0p2, etc. So worst case, I hope I can manually create partitions and move the data around, and get fstab loading it all. Of course updating the efi partiion and grub. But it would be really nice for the original ISO to support this out of the box.

How does one get involved with LinHES development?

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Not using LinHES because 8.6 didn't support my hardware, but still very interested in future developments, so lurking once in a while.