RE: 2nd drive install issues with Dell 9550 (p56f001)
I think you'll find you DO need to disable RAID. The RAID setting is designed to allow the M.2 drive to act as a cache for the hard drive. Without risk to the system, you could:
Remove the M.2 drive.
Set the system for AHCI.
If you then see the hard drive -- which you almost certainly will -- you'll know that in order to use both drives, you must have them set to AHCI (which either means an ground-up reload or a registry tweak to change Windows from RAID to AHCI).