Add HDD Drive to a SSD only iMac 27" 2017

"If you have a SSD only system (PCIe/NVMe blade SSD) then you can add a second drive! This can be either a 3.5"" SATA HDD or a SSHD for a hard drive. Or, it can be a second 2.5"" SATA SSD.

Here's the IFIXIT guide you'll need to follow: iMac Intel 27"" Retina 5K Display Hard Drive Replacement

Now you do need a special SATA cable Retina 5K Hard Drive Combo Cable, Side frames: Retina 5K Hard Drive Mount Left & Retina 5K Hard Drive Mount Right and an in-line thermal sensor and replacement tapes OWC In-line Digital Thermal Sensor for Hard Drive Upgrade for 27"" iMacs 2012 and Later. If you decide to go with a 2.5"" SSD drive you'll need a adapter frame to mount it into the 3.5"" frame.

Now before you go down this path keep in mind you need to use the correct tools and technique to take the display off if not you can easily damage it which will be an expensive oops!

Lastly, an external Thunderbolt RAID drive will be faster and can offer deeper protection of your data so think it though which way is best for you."

"HI Dan, thanks for your informative posts! I was reading specs on Everymac.com and it lists the 2017 27"" iMac OEM SSD in the Primary Storage position (SATA) with the PCIe slot empty (secondary). But from your post it sounds like the opposite, the SSD come in the form of a PCIe ""blade"" with the SATA slot empty?

any clarification would be helpful, as Im buying a used 2017 iMac 27"" 5K and I'd like to upgrade to two internal SSDs, 1st for OS/apps, 2nd for files. And it sounds like getting an OEM 256G blade would be difficult to say the least. (yes I aware of Ext SSD option/warranty/teardown/etc but I need both T3 ports for audio hardware)

many thanks"

Does the harness support a SATA 2700 rpm drives? Or is their one that does?

There is no room for more drives inside the iMac. Some previous models could have the CD removed, but then had slow SATA connection. Best to use a thunderbolt external drive, check out macsales.com