Flash storage cannot be removed from the logic board in the new 27-inch iMac. According to technical documents seen by MacRumors, in the 4TB and 8TB configurations the flash storage expansion board is attached to a connector on the logic board. However, the expansion board and connectors are not present in the 256GB, 512GB, 1TB, and 2TB configurations.
Logic Board and Flash Storage Attached For Encryption Purposes in New iMac
Elsewhere, the documents says that the flash storage and logic board are paired for hardware and encryption purposes. Consequently, data is lost if the logic board is replaced. Customers should back up regularly.
[One New iMac is Fabulous. The Other, Not so Much]