Chosen Solution
I recently purchased a 21.5" 2015 4K Retina iMac from an old friend. It only came equipped with the standard 1 TB HDD but I’d like to replace it for a SSD. I’m wondering if I will find that there is still the Apple Socket for the OEM SSD’s on the back of the logic board, so that I could add one of those (if I can find one) instead of adding a 2.5" SATA SSD? Short question - is the SSD socket on the back of the logic board still present (just empty) on an iMac that only came with a spinning hard drive?
Your systems specs: iMac 21.5" i5" 1.6 GHz (Late 2015) as well as the others in iMac 21.5" 2015 (iMac16,2) line. From EveryMac: This model has a Serial ATA (6 Gb/s) connector for a 2.5" hard drive. If configured with a “Fusion” Drive or an SSD at the initial time of system purchase, it also has a PCIe connector (PCIe 2.0 x4 NVMexpress interface), but this connector is not present if the system only is configured with a hard drive at the time of purchase. So sadly no, you’ll need to swap out the 2.5" HD for a SSD or what I would recommend is think about getting a ThunderBolt2 external SSD RAID drive and make that your boot drive.
