Can't see drives on Dell XPS 8940 with fdisk or gdisk to partition them for Slackware

Hello, I'm trying to install Slackware on a new Dell XPS 8940 desktop. Created a boot image usb stick, logged in as root. But my internal hard drives are not visible to fdisk, gdisk, or lsblk. Only the usb stick drive appears as /dev/sda. Trying fdisk /dev/sdb, etc results in a drive not found error.

The Optical drive isn't visible either. However the disks & optical drive are fine in Windows 10.

Bios info:

Bios version 1.0.1 Build 4400.12 UEFI Rom

Drive info:

SATA-0 HDD 6001 GB WD-WX12040N1SV9
SATA-1 HDD 6001 GB WD-WX12D4073KNY
M.2 PCIe SSD 512 GB CD05N905213805V1L

Other Bios settings:

SATA Operation setting -- RAID on

Secure Boot setting -- not enabled

Fastboot setting -- minimal