The service manual here has the setup information and settings in the BIOS. In the BIOS under the heading of Drives, various drives can be enabled or disabled. Check that setting first. Make sure that you have AHCI enabled or the drives will run in RAID. But be forewarned that if you change the setting in the BIOS Windows will no longer boot. You can change the setting though as described on this page at Top-Password and it will not affect Windows boots.
I purchased a Precision 5820 Tower XCTO Base (Feburary of 2022), that came with four Hard Drive Tray Caddies. The top right caddy has an Intel NVMe PCIe SSD (Front PCIe FlexBay) with a M.2 256GB PCIe NVMe in it. This has been working fine. Today I tried to add two additional sata hard drives. The hard drives work fine in the bay next to the Flexbay (top left); however, the two lower bays don't appear to work, no indicator lights and no detection of drives. Is there something in the bios that needs to be changed so these bays will work?
Dell Precision 5820 Tower XCTO Base, Hard drives not being detected