I've got the Z170 Classified K mobo.
This board didn't support PCIe NVMe drives out of the box, I was only able to use my Intel 1.2 TB Internal SSD - Solid-State Drive 750 Series - PCIe 3.0 x4 (NVMe) drive when BIOS version 1.04 or 2.01 came out. (can't remember which)
Since then I've been using it without issue until yesterday when I turned my PC on in the morning and it wasn't seeing my drive.
I've tried re-seating the PCIe card in it's slot (PCIe Slot 4), I've updated the BIOS to 2.05 to no avail.
When I first flashed the new BIOS it saw the card/drive no problem, correctly identifying it, but when rebooting after flashing the new BIOS version it didn't 'see' the card anymore.
Basically I'm back to where I was when I first bought the Mobo.
The Mobo 'sees' an M2 NVMe drive in the 80mm slot no issue, but not a PCIe version.
I'm aware that to use a PCIe card version I have to ensure that all the M2 slot options are disabled, that's what I had setup previously when it was working fine.
Finding this really frustrating.