4 bays expandable to 9 with the DX517, 2 M.2 NVMe slots.
Six 3.5-inch bays, 2 PCIe slots, up to 64 GB of RAM.
Compact and all-SSD, but capacity is limited by M.2 SSDs.
👍 What we like
- ✓Expandable capacity: up to 9 drives for the DS923+ with the DX517, expansion enclosures for the TS-673A and the Flashstor 6.
- ✓M.2 NVMe slots on all three models, for SSD caching or a fast volume.
- ✓Full-featured operating systems (DSM, QTS or QuTS hero, ADM) with built-in RAID, snapshots and file sync.
👎 What to watch
- ✕The Flashstor 6 only accepts M.2 SSDs internally, which cost more per terabyte than hard drives.
- ✕The DS923+ ships with only two 1GbE ports (10GbE requires an optional module).
- ✕Drives and expansion units come on top of the enclosure price.
🏆 Our picks
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Asustor Flashstor 6 FS6706T
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📑 Contents ▾
Storing several dozen terabytes starts with enough drive slots and room to expand. This comparison covers three expandable NAS units with 4 to 6 slots: the Synology DS923+ (4 bays, up to 9 drives with the DX517 expansion unit), the QNAP TS-673A (six 3.5-inch bays) and the Asustor Flashstor 6 FS6706T (six M.2 NVMe slots, SSD only).
Why this choice matters
The usable capacity of a NAS depends on the number of slots, the size of the drives and the RAID level. RAID 5 uses the equivalent of one drive for parity; RAID 6 uses the equivalent of two. On a 4-bay NAS, RAID 6 therefore sets aside half of the raw space for protection.
Once the internal bays are full, there are two options: replace the drives with larger ones, or add an expansion unit. All three NAS units in this comparison support expansion, but in different ways: an eSATA port and the DX517 unit for Synology, a PCIe card or a USB enclosure for QNAP, USB enclosures for Asustor.
M.2 NVMe slots round things out, as an SSD cache or a fast volume depending on the model. On the Flashstor 6, they are the main storage.
Buying criteria
Slots and drive type: 3.5-inch bays let you use hard drives, which cost less per terabyte than SSDs. A NAS with only M.2 slots requires SSDs.
Expansion: check which expansion unit the manufacturer supports, how many units can be connected and through which interface (eSATA, PCIe card, USB).
Processor and memory: they matter for secondary workloads (containers, virtual machines, photo indexing). The DS923+ uses a dual-core AMD Ryzen R1600, the TS-673A a quad-core AMD Ryzen V1500B, and the Flashstor 6 a quad-core Intel Celeron N5105. Maximum memory ranges from 16 GB (Asustor) to 64 GB (QNAP).
Networking: the DS923+ provides two 1GbE ports (10GbE optional with the E10G22-T1-Mini module), while the other two models have two 2.5GbE ports. The TS-673A can take a 10GbE card in one of its PCIe slots.
Data protection: all three operating systems support RAID 5, RAID 6 and snapshots.
The scores are based on these criteria, applied to the manufacturers’ specifications.
Presentation of the 3 recommended products
1. Synology DS923+
The Synology DS923+ is a 4-bay NAS for 3.5-inch or 2.5-inch SATA drives. It scales to 9 drives with a DX517 expansion unit (5 bays) connected to its eSATA port. It runs on a dual-core AMD Ryzen R1600 (4 threads, up to 3.1 GHz) with 4 GB of DDR4 ECC memory, expandable to 32 GB. Two M.2 2280 NVMe slots can be used for SSD caching or a storage pool; Synology specifies its SNV3400 and SNV3410 SSDs for this. Networking is limited to two 1GbE ports, with a slot for the optional E10G22-T1-Mini 10GbE module. DSM includes Synology Drive for file sync, plus Hyper Backup and Snapshot Replication for backups. Synology replaced it in 2025 with the DS925+ (two 2.5GbE ports, USB-C expansion): check the DS923+‘s availability before buying.
2. QNAP TS-673A
The QNAP TS-673A offers six 3.5-inch SATA bays (compatible with 2.5-inch SSDs) and two M.2 2280 NVMe slots (PCIe Gen3 x1) for SSD caching or Qtier. Its x86 AMD Ryzen V1500B processor (4 cores, 8 threads, 2.2 GHz) comes with 8 GB of DDR4, expandable to 64 GB, with ECC memory support. Two PCIe Gen3 x4 slots can add 10GbE networking or extra M.2 SSDs; with an expansion card, they also connect 8-bay TL-D800S JBOD enclosures (up to two, according to QNAP’s compatibility list). Over USB, the NAS accepts up to two 4-bay TR-004 enclosures. It has two 2.5GbE ports, runs either QTS or QuTS hero, and handles file sync through Qsync.
3. Asustor Flashstor 6 FS6706T
The Asustor Flashstor 6 FS6706T is an all-SSD NAS: its six M.2 2280 NVMe slots (PCIe 3.0 x1 each) replace conventional bays, and it does not take 3.5-inch drives internally. It uses a quad-core Intel Celeron N5105 (2.0 GHz, up to 2.9 GHz) with 4 GB of DDR4, expandable to 16 GB, and two 2.5GbE ports. To add hard drives, Asustor supports up to two USB expansion units (AS6004U or AS5004U, 4 bays each). ADM offers EZ Sync for file sync and EZ Connect for remote access. Internal capacity depends on the size of the M.2 SSDs installed, which still cost more per terabyte than hard drives.
Comparison table
| Product | Synology DS923+ | QNAP TS-673A | Asustor FS6706T |
|---|---|---|---|
| Internal slots | 4 x 3.5/2.5-inch SATA bays | 6 x 3.5/2.5-inch SATA bays | 6 x M.2 NVMe slots |
| Expansion | 9 drives with the DX517 | Up to 2 TL-D800S (8 bays) or 2 TR-004 (4 bays) | 2 AS6004U or AS5004U USB units |
| Processor | AMD Ryzen R1600, 2 cores | AMD Ryzen V1500B, 4 cores | Intel Celeron N5105, 4 cores |
| RAM (stock / max) | 4 GB ECC / 32 GB | 8 GB / 64 GB | 4 GB / 16 GB |
| M.2 NVMe | 2 | 2 | 6 (main storage) |
| Networking | 2 x 1GbE (10GbE optional) | 2 x 2.5GbE | 2 x 2.5GbE |
| Operating system | DSM | QTS or QuTS hero | ADM |
Use cases
The Synology DS923+ suits users who want to start with 4 drives on DSM, then add 5 bays with the DX517 later. The QNAP TS-673A is for those who want the most internal hard drives, memory headroom for virtual machines or containers, and a path to 10GbE through a PCIe card. The Asustor Flashstor 6 FS6706T targets compact SSD storage, for example a working library for video editing, rather than archiving very large volumes.
Pitfalls to avoid
Do not confuse M.2 slots with 3.5-inch bays: the Flashstor 6 only accepts M.2 SSDs internally. Plan for usable capacity after RAID, not raw capacity. On the DS923+, check the compatibility list: Synology only guarantees full functionality with the drives listed there. On the TS-673A, M.2 SSDs are not hot-swappable and QNAP recommends fitting them with a heatsink. Finally, RAID is not a backup: keep a copy of important data on another device.
Verdict
The Synology DS923+ is the most balanced choice in this selection to start with 4 drives and grow to 9. The QNAP TS-673A offers the most internal bays for hard drives and the most upgradeable platform (memory, PCIe slots). The Asustor Flashstor 6 FS6706T is best kept for all-SSD storage, where compactness matters more than capacity.
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