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Just keep in mind these 'Thin Series' systems are a bear to open so make sure you follow the IFIXIT guides to the letter and use the correct tools as you can easily damage the display in the process of taking the display assembly off which would be a very expensive opps! You can then add in the SATA drive later.
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Then if you want to add more storage (HDD or SSD). So I would recommend you get the PCIe SSD based system and I also recommend you get the largest SSD you can get as Apple is still the only source for their custom SSD. This is the case in the 21.5" model as you can see here in the IFIXIT teardown:: iMac Intel 21.5" Retina 4K Display 2017 Teardown So far we don't know if you buy a HDD mode only if the PCIe SSD slot and the sundry parts are even present as Apple has in the past has not populated the logic board. Upgrading the PCIe SSD requires almost completely tearing down the system as seen here: iMac Intel 27" Retina 5K Display SSD Replacement. The new 27" iMac's do have the same access door in the back as the last model, as seen here: iMac Intel 27" Retina 5K Display RAM Replacement.
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