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Hello I want to upgrade my 21.5-inch Imac from 2019, by replacing the 1TB HDD with a PCI Express Type SSD. I read on some website that if my imac was not equipped with a fusion drive at the time of purchase, the pci express port would not be present? Thank you for your return cordially

Sadly, your system only has the SATA interface. The PCIe connector is not present in your system. Only Fusion Drive’d systems or ones which had a SSD only installed. From EveryMac: It 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, but this connector is not present if the system only was configured with a hard drive. You can still install a SSD (2.5” SATA) replacing your HDD.