I bought a second-hand HP Z820 workstation. Upon turning it on for the first time, Windows 10 installed and everything works smooth. The machine has two storage devices: a 512GB SSD (slot 0) and a 2TB HDD (slot 1). It also has two additional empty slots (2 and 3). The SSD is primary and has Windows 10 on it.
I bought a 2TB SSD and a 3.5" caddy for it and tried to add that in slot 2. It does not appear in BIOS or Windows. I also tried to swap slots with the HDD. Now the HDD shows up in slot 2, but slot 1 appears empty.
The disk works. I have a small SATA to USB adapter and I have initialized and read/written to the disk on a different computer. The caddy also works.
Somewhere I read it could be the SATA mode. In the BIOS, SATA mode is set to "RAID+AHCI". I could change that to "AHCI" or "IDE". Should I try that or do you have other ideas?