TerraMaster F4-423 vs F4-421: NVMe slots make the F4-423 a meaningful upgrade

The F4-423 adds two M.2 NVMe slots for caching or storage, while the F4-421 only has SATA bays. Both have four SATA bays for 3.5-inch and 2.5-inch drives, but the F4-423 lets you run an NVMe pool alongside a SATA array.