The bay count sets the arithmetic, the brand sets the extras

The three 2-bay units publish no redundant mode beyond a mirror. The QNAP TS-264 lists Single, RAID 0 and RAID 1. The UGREEN DXP2800 lists Basic, JBOD, RAID 0 and RAID 1. The Synology DS225+ lists Basic, JBOD, RAID 0, RAID 1 and SHR. With two drives, the only redundant option is a mirror, and a mirror keeps one drive’s worth of data. SHR changes which two drives can form that mirror, because Synology’s documentation allows different capacities in one pool, but it does not change the ratio. The usable size is still set by the smaller drive, with unused space left on the larger one.

All four 4-bay units add RAID 5, RAID 6 and RAID 10. The DS925+ also lists SHR and the TerraMaster F4-424 Pro lists TRAID. Across four drives, RAID 5 sets aside one drive’s worth of space for parity and leaves three usable.

Published capacity figures measure different things

Synology publishes a maximum single volume size rather than a raw total: 108 TB on both the DS225+ and the DS925+, rising to 200 TB on the DS925+ once 32GB or more of memory is installed. That is a filesystem limit, not a bay count multiplied by a drive size. The other vendors publish raw totals: 24 TB for the TS-264, 72 TB for the TS-464, 88 TB for the F4-424 Pro, stated as 22 TB x 4, 144 TB for the DXP4800 Plus across its four SATA bays, and 80 TB for the DXP2800 counting its two M.2 slots alongside its two bays.

The raw figures are not comparable across brands. TerraMaster’s assumes 22 TB drives. UGREEN’s 144 TB across four bays works out to 36 TB per bay, and QNAP’s 72 TB to 18 TB. Each is the vendor’s own ceiling as published, not a difference in the chassis, so read them as what the unit is stated to accept rather than as a ranking.

The one capacity figure that grows with hardware rather than with a larger drive is the DS925+’s 180 TB raw with the DX525 expansion unit, which takes the array to nine drives. No other unit in this set publishes an expansion chassis.

Starting a 4-bay with two drives

Two of the four 4-bay vendors document growing an array one drive at a time. Synology’s knowledge base describes expanding an SHR pool either by adding drives to empty bays or by removing the smallest drive, installing a larger one, and repairing the pool before replacing the next, without rebuilding from scratch. Data stays accessible during the process, with a note that connections can be temporarily suspended. Synology’s RAID calculator recommends SHR or SHR-2 when drives differ in capacity, and the DSM help gives the example of adding a 1.5 TB and a 1 TB drive to a pool, with unused space left on the larger drive.

TerraMaster’s TRAID page says you can continuously add hard disks to the array even when capacities differ widely, and describes the in-place procedure: with the unit powered on, remove one drive, insert a larger one, then open Storage Pool in the control panel and choose Repair.

When two bays are genuinely enough

A mirror is enough when the whole library, plus the growth you can foresee, fits on one drive of a size you can buy. The three 2-bay units differ far more in what surrounds the bays than in the RAID list.

The DS225+ ships with 2GB of DDR4, tops out at 6GB through its single SODIMM slot, has no M.2 slots, and pairs one 1GbE port with one 2.5GbE port. The TS-264 and the DXP2800 both ship with 8GB, both reach 16GB, and both carry two M.2 slots. UGREEN lists its slots as usable for caching or storage; QNAP’s page pairs the TS-264’s slots with up to 4 TB of M.2 SSD caching. The TS-264 also has a PCIe Gen3 x2 expansion slot, which neither other 2-bay publishes.

Network: where 2.5GbE stops being the limit

The published throughput figures follow the ports. Synology publishes 282 MB/s read and 217 MB/s write for the DS225+; the test configuration is not stated on the spec page. The DXP2800 publishes up to 312.5 MB/s on a single 2.5GbE port. The TS-264 publishes up to 589 MB/s with both 2.5GbE ports trunked. The DS925+ publishes 522 MB/s read and 565 MB/s write over two 2.5GbE ports. The F4-424 Pro publishes 283 MB/s linear, with link aggregation up to 5Gb combined. The TS-464 publishes port trunking up to 5Gbps and no MB/s figure. The DXP4800 Plus publishes up to 1250 MB/s through its built-in 10GbE port.

Among the other 4-bays, the TS-464’s PCIe slot accepts a 10GbE or 5GbE card later; the DS925+ and the F4-424 Pro publish no 10GbE route.

If part of your library arrives on an external SSD plugged straight into the NAS, the USB generation matters too: the QNAP, UGREEN and TerraMaster units each publish at least one USB 3.2 Gen 2 port at 10Gbps, while both Synologys publish USB 3.2 Gen 1.

Memory, CPU and the app you will actually open

Installed memory runs from 2GB on the DS225+, to 4GB of ECC DDR4 on the DS925+, 8GB on the TS-264, TS-464, DXP2800 and DXP4800 Plus, and 32GB of DDR5 on the F4-424 Pro. Published maximums are 6GB, 32GB, 16GB, 16GB, 16GB, 64GB, and 32GB with no higher figure for the TerraMaster. The DS925+ is the only unit in this set that lists ECC memory, and the DXP4800 Plus has the highest ceiling.

Core counts: four on the DS225+’s Celeron J4125, four on the Celeron N5095 shared by both QNAPs, four on the DXP2800’s Intel N100, four cores and eight threads on the DS925+’s Ryzen V1500B, five cores and six threads on the DXP4800 Plus’s Pentium Gold 8505, and eight on the F4-424 Pro’s Core i3-N305. Every unit publishes a hardware transcoding or encoding line; TerraMaster’s is the most specific, H.264, H.265, MPEG-4 and VC-1 up to 4K at 60fps.

On software, the split is by vendor. Synology names Synology Photos, with facial and object recognition, on both the DS225+ and the DS925+. UGREEN names UGOS Pro’s AI photo album with facial recognition and duplicate detection on the DXP2800, and AI photo management with semantic search on the DXP4800 Plus, which also carries an SD card slot for direct import. QNAP’s spec pages name QTS and QuTS hero and describe real-time transcoding and multi-cloud backup; TerraMaster’s names TOS and its transcoding engine. Neither names a photo-management app on the pages retrieved, which is a gap in those pages rather than a statement that none exists.

The figures a comparison tends to skip

The remaining published figures are the ones a spec comparison tends to skip. Warranty is 3 years extendable to 5 on both Synologys, 3 years on both QNAPs, and 2 years on both UGREENs and the TerraMaster. Noise is published for two units only: 29 to 34 dB in quiet mode for the DXP2800 and 21.0 dB(A) at standby for the F4-424 Pro. Within a brand, the 4-bay is wider rather than taller: the DS925+ is 7.8 in (199 mm) wide against 4.3 in (108 mm) for the DS225+, at 5.0 lb (2.26 kg) against 2.9 lb (1.3 kg), and the TS-464 is 6.69 in wide against 4.13 in for the TS-264. The F4-424 Pro has the heaviest published weight here at 7.5 lb (3.4 kg).