Synology DS420+ vs other NAS setups: SATA storage with NVMe-only cache

The DS420+ uses SATA drives for actual storage in its main bays, while its M.2 slots are strictly for NVMe SSD caching — no way to use them as storage. This hard line between storage and cache is consistent across Synology’s lineup, unlike some competitors that let you use M.2 slots for either purpose.