I have installed Slackware64 15.0 on a new build, successfully completing the setup script but on the reboot, returns to efi screen. I did change the boot priority to hard drive.
I downloaded the iso image from one of the mirrors, burned it to a dvd.

I use a usb dvd drive on the new build. I turned off safe boot, then booted using the usb dvd. I used cgdisk to set up both drives, m.2 and the rotating drives. I have tried putting the efi partition on the m.2, but the install would not boot up, I then wiped both drives and put the efi partition on the rotating drive, also would not boot up.

I got a "live" dvd to boot up so the motherboard options may be correct.
The new build uses a MSI PRO B760M-P DDR4 board, Samsung 980 nvme m.2 500GB and Seagate Baracuda 4TB hard drive.

What do I need to look for?

Thanks in advance

John R

is efi (ef00) partition in first place? FAT32 with boot flag ?
you can check/fix it with gdisk

the efi partition is ef00 and format is fat32

Are you using elilo or grub?

using elilo

Make an EFI/BOOT directory on the efi partition on the m.2 drive, copy all the files from EFI/BOOT/Slackware to the EFI/BOOT directory, rename EFI/BOOT/elilo.efi to EFI/BOOT/Bootx64.efi. If that doesn't work try it on the spinning drives. If it still doesn't boot change over to the grub bootloader.

I made the /EFI/BOOT directory, copied the files from /EFI/Slackware to /EFI/BOOT, renamed elilo.efi to Bootx64.efi.

now the boot starts ...

ELILO V3.16 for EFT/x86_64
.
Loading kernel vmlinuz... done
Loading file initrd.gz... done
(hangs) but it does respond to <ctl><alt><del> boot preference is still dvd, but goes directly to elilo, loads kernel and initrd.gz and hangs

If I can break into EFI and force a boot from dvd or usb stick then I'll wipe it and try again.

Slackware used to install flawlessly before UEFI!

Esc, or one of the f-keys at first turn on should get you to the bios/firmware boot menu. This menu is different from the settings menu on. You may have to use the grub boot loader as elilo dosen’t work on some systems.
Another thought may be a video resolution problem, maybe adding “video=1024x768” or something similar as a kernel option will help

already tried a few times, may need to take a few more tries, the default is "fast boot", I didn't think to disable it!

I am wondering whether you have told the elilo.conf where your root is?

I had a bit of trouble getting use to elilo, but once I got it booting there has been no problem.

I have asked that the configuring your system page be updated to include elilo information.

Incase it might help, here is my elilo.conf file. It's in /boot/efi/EFI/Slackware

Other posters/members might say it is wrong. But while ever it works, I'll not be bothered to fix it.

Yes disable fast boot

I managed to get it to boot from the iso, wiped all partitions and re-installed.
I put the efi partition on the nvme drive, after the setup script, I made the "BOOT" directory and copied the files from the Slackware directory, renamed the elilo.efi to Bootx64.efi. Verified the root and vga normal. then entered reboot.

still stuck after loading vmlinuz and initrd.gz

What else is there to check?

It may be an elilo problem, install grub bootloader:
Boot the iso and manually mount root partition to /mnt, mount boot partition if it exist to /mnt/boot and any other needed partitions, mount efi partition to /mnt/boot/efi
mount --bind /dev /proc /run /sys to /mnt/{dev proc run sys} respectively
chroot /mnt
grub-install
grub-mkconfig -o /boot/grub/grub.cfg
cp /boot/efi/EFI/slackware-15.0/grubx64.efi /boot/efi/EFI/BOOT/Bootx64.efi.
exit
reboot

as a test. I installed Debian 12 successfully, just to confirm there is no hardware problem.
then did a clean, full install of slackware 15.

no luck with the grub-install, something about 'source not specified'

To install grub during slackware installation, at the lilo/elilo configuration prompt open a second console:
continue with configuring the system skip/cancel installing lilo/elilo