The Synology DS1621+ uses SATA drives with NVMe reserved for caching

The DS1621+ takes 3.5-inch and 2.5-inch SATA drives for primary storage — HDDs and SSDs both work. Its two M.2 NVMe slots are strictly for caching, so don’t plan on using them as main storage volumes. It’s a clear separation between what stores your data and what speeds it up.