I already bought 512 GB Samsung Pro 960 M.2 SSD. After I disassembled my iMac 2017 27". I'm shock realizing the interface is different! I feel stupid. I'm ordering the adapter which is about 1 month to arrive. =( I'll buy the adapter to SATA for now and use it as storage. Later on I'll buy the correct SSD and install the OS on it.