Now you can connect 4 disks to Dell's onboard SATA Intel controller. But if you want to use it as e.g. a high performance NAS and run Windows Server 2016 you will probably need more than 4 SATA ports!
In my situation I wanted to install a small Samsung SSD as a boot drive (there is space for it in the case), connect the internal DVD-ROM and also add 4 x 10 TB Seagate Enterprise Capacity (Exos) hard drives for NAS functionality - so a total of 6 drives. Using Windows storage spaces in mirror mode, maybe with ReFS 3.1 should replace a hardware RAID controller and make it even more reliable.
So in order to be able to use 6+ SATA drives I needed an additional PCIe SATA controller; this is even stated in Dell's manual. After a lot of research it turned out that the affordable Syba sd-pex40099 4 Port SATA III PCI-EXPRESS 2.0 x 1 Controller with Marvell 88SE9215 chip from Amazon should work ok with the server so I immediately ordered it. What I received was actually an IOCREST SI-PEX40064 but it looks like this is the same card as the one ordered.
Then I connected my SSD to the first port of the Syba (Iocrest) SATA controller and Dell's DVD-ROM to the second.
Now make sure that there is also at least one drive connected to the ONBOARD SATA controller, otherwise the server won't boot no matter what you do but give you this strange error:
"Alert! Hard Drive or Optical Drive not found." (Thank you Dell for wasting my time before I found out what was causing this...)
1. You can use UEFI so first enable it under Settings -> General -> Boot Sequence in the BIOS
2. In order to be able to boot from the Add-On SATA card you also need to check Enable Legacy Option ROMs under Advanced Boot Options. Enable Attempt Legacy Boot is NOT needed.
3. It looks like you CANNOT use Secure Boot, as this is not usable with the legacy option ROMs setting. But this is not so dramatic I think...
That's it. I hope this post is helpful!
BTW: Performance for the SSD on the add-on SATA controller is quite good for a Samsung 840 Pro SSD: ~ 400 MB/s.