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Can I add a 2.5” SSD in addition to the 1TB PCIe ssd that was original. I want two SSDs in the iMac

Yes! You can add in a SATA SSD iMac Intel 27" Retina 5K Display Hard Drive Replacement. You’ll need this kit iMac Intel 27" (Late 2012-2019) SSD Upgrade Kit as well as installing the HD cable iMac Intel 27" Retina 5K Display SATA Cable Replacement as well as the HD mount screws for the 3.5” drive (adapter frame here for the 2.5” SSD)