Synology DS425+

Length x Width x Height

- Length: the longest side of the device

- Width: the shorter side of the device

- Height: the vertical dimension of the device

The physical weight of the device, typically measured in grams (g) or ounces (oz).

NAS weight is quoted without drives installed. Each 3.5-inch hard drive adds roughly 600 to 700 grams, so a loaded four-bay unit is far heavier than its spec sheet suggests.

Whether the unit is a desktop enclosure meant to sit on a shelf or a rack-mounted chassis measured in rack units (1U, 2U).

Desktop models are quieter and easier to place in a living space. Rack-mount models fit a 19-inch cabinet alongside networking gear but usually run louder fans.

The specific processor installed in the NAS, such as an Intel Celeron, an Intel Core i3, an AMD Ryzen Embedded, a Realtek RTD or a Rockchip ARM chip.

The CPU sets the ceiling for everything beyond plain file sharing: container workloads, virtual machines, on-the-fly media conversion and photo indexing all scale with it.

The number of physical processing cores available to the operating system.

More cores let the NAS keep serving files while it also runs containers, indexes photos or handles a backup job, instead of stalling under a single heavy task.

Whether the processor is x86-64 (Intel or AMD) or ARM (Realtek, Rockchip, Annapurna).

This is the single biggest compatibility divide in NAS software. Most container images, virtual machine support and third-party packages target x86-64, while ARM models are cheaper and far more power efficient but run a reduced app catalog.

An integrated GPU that can convert video formats in dedicated silicon rather than on the main CPU cores, for example Intel Quick Sync Video.

Without it, a media server has to convert streams in software, which pegs the processor and often fails on 4K. With it, a low-power chip can reshape several streams at once while barely raising power draw.

Note that a fast CPU does not imply transcoding: several AMD Ryzen Embedded NAS have no integrated graphics at all.

A dedicated instruction set (AES-NI) that performs encryption and decryption in the processor itself.

Encrypted shared folders on a NAS without this engine can lose most of their transfer speed. With it, encryption costs very little performance.

A dedicated AI accelerator, quoted in TOPS (trillions of operations per second), used for local machine learning work.

On a NAS the NPU handles face and object recognition in photo libraries and local semantic search, keeping that processing on the device instead of sending images to a cloud service.

The amount of RAM the NAS ships with, measured in GB.

File sharing alone is happy with 2 GB. Running containers, a media server or virtual machines is where memory becomes the bottleneck, and the Btrfs and ZFS file systems also use free RAM as a read cache.

The largest amount of RAM the machine supports after an upgrade.

Entry-level NAS often solder their memory, so the shipped capacity is also the ceiling. Being able to go from 4 GB to 32 GB or 64 GB later is what keeps a NAS useful as you add services to it.

The memory technology used, such as DDR4, DDR5 or LPDDR4X.

DDR5 offers more bandwidth than DDR4, while soldered LPDDR variants are common on compact ARM models and cannot be replaced.

Error-correcting code memory detects and repairs single-bit errors before they reach your data.

For a machine that stores files for years and runs unattended, ECC removes one route by which silent memory corruption can be written to disk. It is usually reserved for higher-end and business-oriented models.

The number of internal bays for 3.5-inch hard drives or 2.5-inch SATA drives.

Bay count decides both capacity and which redundancy levels you can use. Two bays only allow mirroring, four bays open up RAID 5 and RAID 6, and more bays let you lose a drive without losing the array's usable space.

The ability to remove and replace a drive while the NAS is running.

When a drive fails in a redundant array, hot swap lets you slot in the replacement and start rebuilding without downtime. Budget models often require a full shutdown and, on some, opening the case.

The number of M.2 slots for NVMe solid-state drives, normally in the 2280 size.

NVMe drives are used as a read and write cache in front of the spinning disks, which mainly speeds up thumbnail generation, database work and browsing large photo libraries.

Whether the NVMe slots can hold a normal storage volume instead of only acting as a cache.

This is the difference between an all-flash share you can actually store files on and a cache that merely accelerates the hard drives. Several makers restricted their M.2 slots to caching for years before allowing full volumes.

Support for an external expansion chassis that adds more drive bays to the same system.

An expansion unit lets you grow past the built-in bay count without migrating to a new NAS, though the enclosures are proprietary and connect over a dedicated eSATA or USB-C link.

The largest total capacity the internal bays support, counted before any redundancy is applied.

Raw capacity is not usable capacity. A four-bay NAS holding 4 x 20 TB is 80 TB raw, but in RAID 5 one drive's worth is spent on parity, leaving roughly 60 TB to actually store files on.

The number of standard Gigabit Ethernet RJ45 ports.

Gigabit caps real transfers at roughly 110 MB/s, which a single modern hard drive already saturates. It is fine for backups and document shares but is the limiting factor for video editing or fast bulk copies.

The number of 2.5 Gigabit Ethernet RJ45 ports.

2.5 GbE has become the practical baseline for new NAS: it roughly doubles Gigabit throughput, runs over the Cat 5e cabling you already have, and is now common on consumer switches and motherboards.

The number of 10 Gigabit Ethernet RJ45 ports.

10 GbE is worth it when the array itself can feed it, which in practice means several drives striped together or NVMe volumes. It also requires a 10 GbE switch and adapters at the other end.

The number of SFP+ cages, which take a fibre or direct-attach copper module instead of an RJ45 plug.

SFP+ is the form 10 Gigabit takes on rack gear. It runs cooler and further than 10GBASE-T, but needs a matching module and a switch with SFP+ cages.

Bonding two or more network ports into a single logical link.

Aggregation raises total bandwidth across several simultaneous clients and can keep the NAS reachable if one cable or switch port dies. It does not make a single transfer faster than one port's speed.

The ability to power the NAS on remotely by sending a packet over the network.

Combined with a shutdown schedule, Wake on LAN lets a NAS stay off most of the day and come up only when you need it, which cuts both the power bill and the noise.

The total number of USB Type-A ports on the enclosure.

USB ports are used for external drives, one-touch backups and connecting an uninterruptible power supply so the NAS can shut down cleanly during an outage.

A USB Type-C connector, typically running at 10 Gbps.

Beyond fast external storage, some makers use the USB-C port to attach their proprietary drive expansion units.

A USB4 or Thunderbolt port capable of 40 Gbps.

This allows a direct computer-to-NAS link that bypasses the network entirely, which is the fastest way to move a large video project and is mostly found on enthusiast and creator models.

A video output for connecting the NAS straight to a monitor or television.

HDMI turns the NAS into a local media player or a console for the operating system. Most of the established makers omit it deliberately, since their software is managed entirely through a browser.

A built-in card reader for offloading photo and video cards.

For photographers this removes a step: cards can be copied to the array automatically without a computer in the middle.

Striping with distributed parity across three or more drives, surviving the loss of any one drive.

RAID 5 is the usual choice for a four-bay NAS because it spends only one drive on redundancy, leaving three quarters of the raw capacity usable.

Double parity across four or more drives, surviving the loss of any two.

RAID 6 matters most with large drives, because rebuilding a big array takes many hours and a second failure during that window would otherwise destroy the whole volume.

A maker's flexible RAID mode, such as Synology Hybrid RAID (SHR) or TerraMaster TRAID.

Classic RAID wastes the difference when drives are of unequal size. Hybrid modes reclaim most of that space and let you grow an array by swapping in bigger drives one at a time.

A copy-on-write file system with checksums on both data and metadata.

Btrfs is what enables instant snapshots, file self-healing on a redundant array and cheap replication to a second NAS. The alternative, ext4, is faster on weak hardware but offers none of that.

A copy-on-write file system and volume manager built around end-to-end data integrity.

ZFS is the strongest option against silent corruption and offers deduplication and its own snapshot and replication tools, but it is memory hungry and is normally offered as a separate firmware edition.

Point-in-time copies of a shared folder that can be restored in seconds.

Snapshots are the practical defence against ransomware and against deleting the wrong folder, because they take almost no space and cannot be altered by a client that only has network access to the share.

The Network File System protocol, the native way Linux and Unix clients mount a remote share.

NFS is also what hypervisors and many self-hosted services expect. Windows and macOS are served by SMB instead, which every NAS supports.

Presenting NAS storage as a raw block device that a client treats as a locally attached disk.

iSCSI is what virtualization hosts and some backup software need. It is often the dividing line between a maker's entry range and its plus range.

Incremental file synchronization to or from another server, copying only the parts that changed.

Rsync is the standard way to keep an offsite copy on a second NAS, and it is scriptable, which matters if you want backups you control rather than a vendor's cloud.

The software the NAS runs, such as Synology DSM, UGREEN UGOS Pro, QNAP QTS, TerraMaster TOS, ASUSTOR ADM or Ubiquiti UniFi Drive.

The operating system, not the hardware, is what you live with every day. It decides the app catalogue, how backups and snapshots are managed, and how long the unit keeps receiving security updates.

The ability to run Docker or OCI containers directly on the NAS.

Containers are how most self-hosted software is distributed now, including Home Assistant, ad blockers, download clients and databases. This is the feature that turns a NAS into a home server.

Support usually tracks the processor: x86-64 models almost always have it, while entry-level ARM models frequently do not.

Running complete guest operating systems, such as a Linux or Windows VM, on the NAS.

Virtual machines give a service its own kernel and full isolation, which containers do not. They need a fair amount of RAM, so check the maximum memory capacity before planning around them.

An official Plex Media Server package for the NAS platform.

Plex indexes your films and shows and streams them to phones, tablets and televisions. Pair this with hardware transcoding if you plan to watch outside the house or on devices that cannot play the original file.

A built-in network video recorder application that records IP cameras to the array.

This lets the NAS replace a dedicated NVR and keep footage local instead of on a camera maker's cloud. Check how many camera channels are included, since several makers ship only a couple of free licences and charge per extra camera.

Local face, subject and scene recognition across your photo library.

This is what makes a NAS a credible replacement for a cloud photo subscription: searching for a person or a place works without any image ever leaving the house.

Built-in synchronization or backup to cloud services such as Google Drive, Dropbox, OneDrive, Backblaze B2 or Amazon S3.

A NAS alone is not a backup, because a fire or a theft takes every copy at once. Sending an encrypted copy offsite is what completes the rule of keeping three copies on two kinds of media with one of them elsewhere.

An agent that backs up whole computers to the NAS, with bare-metal restore.

This goes further than a shared folder that people copy files into: it images the machines on your network on a schedule, so a dead laptop can be rebuilt rather than just re-downloaded.

A front-panel button that copies an attached USB drive or memory card to a preset folder.

Useful for importing camera cards or handing a drive to someone who should not need an account on the NAS to drop files on it.

The maker's relay service for reaching the NAS from outside the house without configuring your router, such as Synology QuickConnect or UGREEN Link.

This avoids opening ports to the internet, which is the most common way a home NAS ends up exposed. The trade-off is that the connection is brokered through the vendor's servers.

Dynamic DNS keeps a fixed hostname pointing at your home connection even when the provider changes your IP address.

It is the alternative to a relay service for direct remote access, and it is also what lets you obtain a proper certificate for the NAS.

Running a VPN server on the NAS itself, over WireGuard, OpenVPN or L2TP/IPSec.

A VPN puts your phone or laptop back on the home network from anywhere, which is a far safer way to reach the NAS and everything else at home than publishing individual services to the internet.

A command-line shell on the NAS over SSH.

SSH is how you script jobs, inspect what the system is really doing and run tools the graphical interface does not expose. Locked-down appliances often withhold it.

Typical draw in watts while the drives are being accessed, as quoted by the maker.

A NAS runs all year, so the gap between a 15 W unit and a 40 W one is a real line on the electricity bill. Makers also publish a much lower figure for disk hibernation.

Sound pressure in dB(A) measured with drives installed, as quoted by the maker.

Below roughly 20 dB(A) a NAS disappears into a living room. Rack-mount chassis with small high-speed fans can pass 40 dB(A) and belong in a cupboard or a garage.

The drives you fit matter as much as the enclosure: solid-state drives are silent, while high-capacity 7200 rpm hard drives add their own seek noise.

A schedule that powers the NAS down and back up at set times.

Shutting the unit overnight cuts power use and wear, and it pairs with Wake on LAN so the machine can still be brought up on demand.

Communication with an uninterruptible power supply over USB or the network, so the NAS shuts down cleanly before the battery runs out.

Losing power mid-write is one of the few things that can damage an array rather than just interrupt it. UPS support turns an outage into a graceful shutdown instead of a rebuild.