Please can you tell me if the Crucial memory (CT2K4G4SFS824A 8 GB) is compatible with the TS-251D I can't find it on the Crucial web site
Yes, this is a 2x4GB set and it would work. You can also find a compatible RAM list here https://nascompares.com/ram/qnap-ts-251d-compatible-ram-memory-upgrade/I hope this helps.