Apple’s refreshed 27 inch iMac has new hardware additions, which brings some restrictions to the table.
Notably, the T2 chip used in flash storage means storage upgrades will not be possible. The hardware takes care of encryption and computer security. However, since the SSD is connected to the logic board changing it or the logic board will erase all data.
Either the 4TB or 8TB SSD configuration will have the hardware affixed to the board via connector. It’s also worth noting that the SATA port, which was used in the 2018 model is not present in the new iMac.
Flash non-upgradeability is nothing new. Apple has used similar technology in the past for its Mac Pro, iMac Pro and MacBook Pro. The 27 inch version can be upgraded to 128GB RAM, 8TB of storage and the 5000 series graphics by AMD. Customers can upgrade to get the nano-texture glass similar to what the Pro Display XDR has.