Chosen Solution
The iMac 27" 5k 2017 comes with both fusion drive (small m2 like SSD and a HDD) and only SSD. Is the full SSD placed in the m2 slot or the 3,5" HDD slot? So can you buy a iMac 27" 2017 with a 512SSD and put a 6TB HHD in it?
If you buy a new Apple system with only a SSD you are getting a PCIe blade drive which sits on the backside of the logic board. So yes you could add in a 3.5" HDD. But! I recommend you don’t! Keep in mind you’ll void your warranty coverage and if you’ve never taken the display off one of the newer ‘Thin Series’ you can damage it in the process if you don’t use the correct tools and technique. Many people have messed up their displays which would be an expensive Opps! Now lets look at throughput! Your HDD drive’s interface is only SATA III (6.0 Gb/s). Using the Thunderbolt port with a RAID’ed external drive could be over three times faster! In some ways even faster than your internal blade SSD! I would recommend getting the largest SSD drive’d system you can afford from Apple! Then look at getting an external RAID drive if you have lots of stuff!
