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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).

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