As above, SATA controller was set to RAID mode in BIOS and two drives were connected to the SATA controller, however Ctrl + I during POST did not open Intel RAID Option ROM.
This was resolved by following actions in BIOS:
- disabling Secure Boot Mode
- enabling Legacy Option ROMS
Once this was done, Ctrl + I opened Intel RAID Option ROM, and the disks were successfully reset to non-RAID.