How did you make the USB install disk?
You could try making it manually as follows. Create a GPT partition and format it to FAT32 (use the the mkdosfs utility to format if you are using another Slackware install). This will serve as your EFI System Partition. It must be big enough to hold the Slackware install media's huge kernel, initrd and a boot loader (50Mb should do it). The EFI System Partition must be marked as such via the partitioning tool (e.g. with gdisk set the partition type code EF00).
Once the system partition has been created add a ./EFI/BOOT/ directory structure to this partition, into which you place the elilo binary named as BOOTX64.EFI. This exact naming (./EFI/BOOT/BOOTX64.EFI) is not an absolute requirement. You can use a different directory structure and name for your bootloader, however conforming to this suggestion will simplify things for you as UEFI systems consider a bootloader named in this way to be the default.
Into the same directory on your system partition (./EFI/BOOT) copy over the following two files from the official install media: kernels/huge.s/bzImage and isolinux/initrd.img (just the files, not their directory structures). Plus create an elilo.conf, that looks like this:
Code:
prompt
timeout=50
default=install
image=bzImage
label=install
initrd=initrd.img
append="load_ramdisk=1 prompt_ramdisk=0 rw printk.time=0 SLACK_KERNEL=huge.s"
You should now have a USB disk that you can boot from, though you may need to configure your UEFI to consider this the first boot disk (refer to your manufacturer's documentation on how to do this) or use the EFI Shell to specify this disk and/or bootloader.
In addition to the system partition you may wish to have a second partition that holds the rest of the Slackware installation files. This can be formatted with any filesystem supported by the the huge.s kernel. This partition can be mounted after you have booted from your install media and used as a source of packages during installation. Alternatively you can insert an official Slackware CD or DVD once setup is up and running, or perform a network installation.
