As expressed here: 21.5” iMac 21.5 (4K, 2019)
“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.”
Sadly, Apple did you in. You could upgrade the SATA drive to a SSD. Frankly, I recommend going with an external Thunderbolt drive. To get the best performance set it up as your boot drive! Hint! A RAID’ed SSD is faster than the internal PCIe/NVMe drive.
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