So you can now put an unsigned EFI shell as \EFI\BOOT\BOOTX64.EFI onto an FAT USB drive and successfully boot it? If that works, you could launch your Slackware installation from there.

@AlleyTrotter. Thanks, but unfortunately I cannot do this. My UEFI firmware would allow me to start an EFI shell, if there was one installed in my EFI partition (or somewhere in a reachable partition). But as I am unable to boot my installation media in UEFI mode, the EFI partition never gets populated, and thus I have no EFI shell that I could invoke to follow the steps you recommend. Thanks, anyway!
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The 'Shellx64.efi' must be downloaded into to the root of your efi partition.
The shell has many commands which may help to understand your predicament.
After that there should be an option for it in UEFI screens which will allow you to boot the shell in UEFI mode.
John

1 members found this post helpful.

I had problem with ASUS UEFI firmware too, it didn't see the HD EFI partition (ef00) after Slackware install.

Luckily, I did an USB boot key during install so I rebooted with the plugged key and it see the EFI software on the usb key, then it booted and I was able to add the HD EFI partition with efibootmgr. After that, no problem to boot with HD efi software

@gargamel Do you have an usb key?

Good news! Getting some sleep seemed to help.

I did:

Prepare a new USB installation medium
I used Rufus to transfer the Slackware64-14.1-install-dvd ISO image to the USB stick. The only settings in Rufus I changed were for the target file system, which I set to FAT32, and the source file, where I selected the ISO image. Everything else was left at default.

Prepare the computer: Set up UEFI, disable "Secure Boot", and power-off the system
Set up the UEFI firmware, so that UEFI is enabled, but "Secure Boot" is diabled. In the case of my ASUS firmware this requires only one modification, which is to "clear" the keys for "Secure Boot". This implicitly disables "Secure Boot", but the keys are still loaded. If you have a similar firmware, leave the OS setting at "Windows 8"; changing this to "Other OS" would disable UEFI. If you want to install and boot your Slackware system in "legacy/BIOS" mode, however, this would be just what you want, of course.
Not sure, if it is strictly necessary, but I decided not only to reboot, but to power-off and restart the machine "cold", in order to make sure that no keys or other traces of "Secure Boot" remain in the RAM.
BTW, with "Secure Boot" enabled, you will get a useful error message that some "unauthorized changes" have been recognised, when you try to boot your installation system from the USB stick. In that case just reboot and disable "Secure Boot". So if your installation medium is intact you have a chance to know, why UEFI boot doesn't work as expected.

Boot the installation system from your USB stick
Restart the machine, and if all goes well, you will be presented with a GRUB screen which allows you to select your installation system to boot with. This will bring you to the usual system prompt where you can log in as root, partition your hard disk(s) and run setup to begin your installation of Slackware on your machine.

So, BIG THANKS to you, guys: Dugan, Didier, Rob, jtsn, AlleyTrotter
Without your endless patience and readiness to delve into the nasty details of UEFI etc. I would never have managed to solve this --- you really saved my weekend!

gargamel

EDIT 9th November, 2014
P.S.: Later I ran into another small problem: The Slackware installer didn't "see" the USB stick as a proper DVD drive and could not use it as the source for Slackware packages. Of course, the easy fix was to insert the 'real' Slackware installation DVD and use that.
So the USB stick provided the installation system, and the packages were taken from the DVD.
I think it should normally work with just the USB stick, but I didn't have the nerves to do further research and tests on that.

Just curious, what is your Asus motherboard?

In my Z87 plus, the Other OS settings let me boot in EFI mode

On my ASUS board, "Other OS" still leaves "Secure Boot" enabled, and the "help" is pure gibberish. They apparently make no distinction between UEFI and Secure boot. It's a RIVE with bios 4804.

However, the way it's currently set I boot into Slackware just fine (apparently in UEFI mode, I run grubx64.efi) but can't boot an efi shell from the bios screen because a message pops up saying "Secure boot is enabled". ASUS is just plain weird.

B85M-G.

It seems they implement this stuff differently, which is weird, as standardising it would be more economically for them. #

For my B85M-G the following seems to be the case:

OS type

• Choose Windows 8 for UEFI with or without Secure Boot
• Choose Other OS for lecacy/BIOS mode

Keys

• If loaded, Secure Boot is enabled
• Clear to disable Secure Boot

When keys are cleared AND OS type is Windows 8, the system will try to boot in UEFI mode "insecurely".

This is really full of good stuff. Wish I had read it before I updated my MB, but, as I said, I was snoozing and completely missed the seriousuness of the BIOS/UEFI transition.

Sorry if I took any thunder, dugan. I meant my post to be more of a , "Hey everybody, check out rEFInd." plug. It just somehow morphed into a UEFI V BIOS thread.