How to choose a NAS: HDDs, M.2, UPS, and which brand

How to choose a NAS starts with a blunt question: where do your important files live if the cloud suddenly turns them into zero-byte names while support “looks into it” for months? A single copy in someone else’s datacenter is hope, not a storage plan.

Below is a practical home or small-shop NAS buying guide: the chassis, HDDs, M.2 slots, and a real UPS. Plus the operational traps that kill arrays even when people “followed the manual.”

This is not an ad for one model. It is a checklist: what to buy, how not to mess up drives and power, and why RAID is not a backup. For copies outside one box see our sync guide Syncthing. For media on your own gear — Jellyfin.

Why a NAS if you already have Google Drive or Dropbox

Cloud is great for phone access and sharing. It does not have to be your only copy. Terms of service almost always limit liability for data loss, while marketing still shouts “secure.”

• Your own perimeter — files and backup rules stay with you.
• LAN speed — photos, video, laptop backups without plan caps.
• Services next door — Photos, Drive, Docker, Jellyfin, camera backups.
• Paired with Syncthing — a second PC or NAS holds a copy without “cloud as the only truth.”

A NAS alone does not make data immortal. It gives control. Control without a UPS, without watching drives, and without a second copy is just an expensive shelf.

How to choose a NAS: what to check first

Bay count and room to grow

Homes usually buy 2 or 4 bays. Two is the minimum for a mirror (RAID 1). Four lets you survive two drive failures (RAID 6 / SHR-2) or keep a hot spare. Eight bays make sense when capacity and budget are already “server-ish.”

Leave spare slots: adding drives later is easier than replacing the whole chassis in two years.

Software beats raw hardware

Synology’s strength is DSM: clear UI, mobile apps, snapshots, replication, Active Backup. QNAP often gives more hardware per dollar and more flexibility, with a different UI. For “store files and don’t suffer,” people often pick Synology; tinkerers also look at QNAP and DIY TrueNAS.

Recent Synology models changed drive policies. The “25+” line pushed proprietary HDDs and NVMe for a while. Older boxes like the DS923+ usually take third-party NAS HDDs calmly. Before you buy, check the Synology compatibility list and DSM version — rules shift within a year.

CPU, RAM, networking

Job	What matters
Files, photos, laptop backups	Mid Plus-class is enough; 4 GB RAM with an upgrade path
Many containers / VMs	Stronger CPU, more RAM, ECC if the model supports it
Plex with transcoding	Intel Quick Sync; plain AMD Ryzen Plus boxes often lack it
Heavy network load	2.5 GbE or better, or a 10 GbE slot

Real Synology examples: the four-bay DS923+ is a workhorse for files and apps; the newer DS925+ is stronger on CPU, but read HDD and M.2 limits for that firmware. Below: TerraMaster, Ugreen, and DIY TrueNAS if DSM is not dogma.

Platform options: Synology, TerraMaster, Ugreen, TrueNAS, and Xpenology

There is no single “best NAS.” There are trade-offs: polished software, hardware per dollar, or freedom to install your own OS.

If you want…	Look at…
Plug-and-play apps	Synology (DSM)
More hardware per dollar; other OS OK	TerraMaster or Ugreen
ZFS and serious control	TrueNAS SCALE (appliance or DIY)
You already have a PC/server	DIY TrueNAS
“DSM on my own hardware”	Xpenology — carefully; see below

Synology — when the ecosystem matters

DSM is still the benchmark for a “friendly NAS”: Drive, Photos, snapshots, mobile clients, Active Backup. You pay for the chassis and sometimes drive/NVMe lock-in.

• DS923+ — proven four-bay for files and services; third-party HDDs are usually fine.
• DS925+ and the newer line — stronger CPU; verify HDD and especially M.2 policy before buying.
• Pick Synology if you want apps, not a terminal. Skip it if the brand eats the whole budget and leaves no money for drives and a UPS.

TerraMaster — hardware and RAM headroom

TerraMaster (e.g. F4-424) is often “lots of NAS for the money”: Intel N-series with hardware transcoding, 8 GB DDR5 out of the box, and a higher RAM ceiling than many peers (about 32 GB on the F4-424 — check the revision). Stock TOS is less mature than DSM.

• Pros: price/hardware, TRAID with mixed sizes, many models welcome TrueNAS/Unraid.
• Cons: TOS polish and calm support trail Synology; some buyers treat the box as a chassis for another OS.
• Worth it if you accept TOS — or install TrueNAS and treat TerraMaster as a backplane in a nice case.

Ugreen NASync — mid-tier value and UGOS

Ugreen NASync (DXP4800 / DXP4800 Plus) arrived hard: N100 or stronger N-series, dual 2.5 GbE, M.2 slots, HDMI on some SKUs, stock UGOS Pro. Hardware per dollar often beats Synology at the same bay count.

• Pros: strong value; BIOS is usually open for another OS (Ugreen will not support third-party OS — know that).
• Cons: UGOS is young; long-term update and security history is still being written. Buying “only for UGOS like DSM” is early.
• Sensible path: Ugreen as chassis + TrueNAS SCALE or Unraid for ZFS and control — or UGOS if you accept a newer vendor OS.

DIY TrueNAS — maximum control

TrueNAS (look at SCALE with Linux apps, or CORE on FreeBSD for the job) is not a living-room app store. It is storage with ZFS: checksums, snapshots, scrubs, clear pools.

Two paths:

• Ready chassis (TerraMaster, Ugreen, sometimes an old PC) + TrueNAS instead of the stock OS.
• Full DIY: board with proper SATA/HBA in IT mode, ECC RAM if you can, separate boot disk, HDD shelf, decent PSU, and the same mandatory UPS.

DIY pros: any hardware, real ZFS, Docker/VMs without a vendor storefront, upgrade by part. Cons: you pick the HBA, you own updates, SMART, and scrubs.

Extra DIY risk: incompatible parts, or junk components — a cheap no-name Chinese motherboard with weak VRM, weird BIOS, and sketchy SATA bridges. Same for anonymous PSUs, risers, and “RAID cards” with cache and no battery. A Synology/TerraMaster/Ugreen box already validated board–backplane–power–cooling. In DIY that compatibility is on you. Saving on the board and PSU is the most expensive way to “save” on a NAS.

For “just photos and laptop backups,” DIY is often overkill. For “I want to know where the bits live and ignore vendor drive politics,” it sits next to NAS + Syncthing as a calm long-term option.

Xpenology — DSM on foreign hardware, with caveats

Xpenology (loaders like ARPL/RR) tries to run DSM on DIY or non-Synology boxes. On paper: familiar UI, cheaper iron.

In practice I do not prefer this path. Original Synology or TrueNAS is calmer. Xpenology keeps hitting everyday pain: codecs, transcoding, and reliably opening iPhone photos and video (HEIC/HEVC and related Photos/File Station flows) — broken, half-fixed, then broken again after a loader update. Add DSM license gray area and no real DiskStation support.

Want DSM? Buy Synology. Want hardware freedom and predictable storage? TrueNAS. Saving on the chassis for “almost DSM” usually costs more time than it saves.

How not to get lost picking a brand

• Minimum fuss → Synology.
• More CPU/RAM/network per dollar → TerraMaster or Ugreen.
• ZFS and long-term control → TrueNAS on a chassis or DIY.
• “DSM on DIY” → Xpenology is possible; for codecs and iPhone media I would still pick real Synology or TrueNAS.
• Budget order: chassis → CMR drives → UPS → then M.2 extras.

Choosing HDDs for a NAS: half of reliability

A chassis without the right drives is a pretty box. Arrays want CMR NAS-class drives rated for 24/7 and vibration from neighbors.

Lines worth buying

• Seagate IronWolf / IronWolf Pro — common under Synology and QNAP. Pro = higher workload and longer warranty.
• WD Red Plus / Red Pro — proper CMR. The word Plus or Pro matters.
• WD Gold — more “enterprise”; sometimes used as a same-size spare.
• Plain WD Red without Plus — SMR risk. Bad for RAID rebuilds. Skip for arrays.

Capacity: home sweet spot is often 8–16 TB per drive. Count usable space after RAID and 2–3× growth, not raw marketing TB.

Batches, age, and why “replace everything at once” is a bad idea

HDDs have no timer that kills the whole set in one minute. But drives from the same batch and age often wear into the same period of the array’s life — not the same second, the same bad year. That is enough for RAID pain.

Practical habits:

• Mix vendors or at least purchase dates at the same capacity (IronWolf Pro + WD Red Pro, etc.).
• Do not swap every disk for new ones in one go. Some older healthy drives + some new ones reduce shared wear.
• On an old server it can be wise to replace part of the set early before full array degradation — especially if they are the same age.
• Watch SMART, temperature, and DSM/QNAP warnings. Replacing one or two hot-swap drives is doable; “all at once” is an outage.

RAID buys time to replace a disk. It does not replace a separate copy elsewhere.

RAID: what to pick and where people go wrong

Layout	Survives	Note
RAID 1 / mirror	1 disk	Simple on 2 bays
RAID 5 / SHR-1	1 disk	Risky on large disks during rebuild — one unread error can abort rebuild
RAID 6 / SHR-2	2 disks	Normal for big disks in 4+ bays
RAID 10	up to half*	*if failures are not in the same mirror pair

“RAID 6 and three failures tend to zero” sounds comforting and is dangerous: failures are not always independent. While the array is degraded, spare margin is gone. Rebuild on large disks takes hours or days and reads every remaining disk end to end. RAID 6 survives a second failure; a third in that window does not. Power spikes, heat, and controller faults count too.

A hot spare is good insurance on RAID 1/5/6. It does not replace a UPS or an off-box backup.

M.2 in a NAS: cache, pools, and limits

Modern NAS M.2 NVMe slots are used differently:

• Read/write cache — speeds hot small files and metadata. Does not grow visible capacity.
• Separate storage pool — on some models a fast volume for VMs, Docker, DBs. Small and expensive per TB — not an HDD shelf replacement.
• Vendor-only NVMe — some Synology units still gate storage pools to certified M.2 even after third-party HDDs returned. Check the NVMe list before buying.

For a typical home file NAS, M.2 is often a bonus (cache), not the foundation: HDD shelf + UPS + second copy. There is also another class — NAS arrays built only from M.2, covered next. Do not spend the whole budget on cache if you need a large cold archive on spinning disks.

NAS on M.2-only RAID — no HDD, no “big” SATA SSDs

Another scenario: the box has no 3.5″ HDD bays (or you refuse to use them), and the array is only M.2 NVMe — mirror, RAID 5/6-style, or a ZFS pool of sticks. Classic 2.5″ SATA SSDs may be absent too — everything sits in board slots.

People do this for speed and silence: photos/VMs/Docker, hot working sets. Capacity is usually smaller and costlier per TB than IronWolf on a shelf. It is not a drop-in replacement for an archive HDD NAS.

Where you see it

• All-flash / NVMe NAS — compact boxes with several M.2 slots.
• 4-bay with many M.2 — NVMe-only pool; HDD bays empty or cold tier.
• DIY / TrueNAS — multi-M.2 board or NVMe backplane; mirror/RAIDZ with no spinners.

What to watch on M.2 RAID

• Cooling — NVMe under write heat throttles and errors without heatsinks and airflow.
• PCIe lanes and bridges — not every slot is x4; cheap boards put some M.2 behind a chip.
• TBW — SSD wear differs from archive HDD; pick models with headroom for constant camera/DB writes.
• Synology compatibility — some models only allow vendor NVMe in pools.
• UPS still mandatory — power loss mid-write hurts NVMe arrays too.
• Second copy — fast M.2 RAID does not replace backup.

Short: HDD NAS = capacity and archive; M.2-RAID NAS = speed and compactness. Mixing tiers in one head is fine; substituting one for the other “because SSD is safer” is not.

UPS for a NAS: do not power it without one

Blunt: do not run a NAS without a UPS. Sudden power loss is one of the fastest ways to get a broken array, an aborted rebuild, and “why won’t the volume mount.”

What matters:

• Headroom — NAS + drives + router/switch if you keep them up. A tiny 350 VA APC is often weak for a modern four-bay Synology; in real use CyberPower-class 1.5–2 kVA is calmer if other gear shares the line.
• Pure sine — better for NAS PSUs and drives on battery.
• USB/LAN signaling — so DSM/QNAP can shut down cleanly before the battery dies.
• Batteries age — replace every few years. If the UPS board fails or the pack will not hold charge, do not wait. It is a consumable, like drives.

A UPS does not make RAID immortal. It removes the dumbest failure class — a breaker trip mid-rebuild.

Operations: a short checklist

1. Once a month — glance at drive health and temperatures in DSM/QNAP.
2. After a SMART warning — plan replacement; do not wait.
3. Do not start a long rebuild in heat or without a UPS.
4. Keep at least one copy of important data off this chassis (second disk, second NAS, Syncthing, offline copy).
5. Test restore: open a file from the copy.
6. Treat the UPS battery as seriously as the drives.

A stack that works in practice

Working pattern for important files:

• NAS (Synology, TerraMaster, Ugreen, or TrueNAS) with RAID 6/10 / equivalent or at least a mirror — primary volume.
• UPS with clean shutdown.
• Drives CMR NAS-class, mixed age/brand, SMART watched.
• Second copy — Syncthing to another machine/NAS or regular offsite backup.
• Cloud — a convenient third channel, not the only one.

That is the sober take after “40% of cloud files vanished” stories and arrays that died mid-rebuild: not one magic brand, several boring layers of protection.

Bottom line

How to choose a NAS is choosing an ecosystem (Synology / TerraMaster / Ugreen / TrueNAS) and bay count. Choosing HDDs decides whether the array survives a couple of years calmly. M.2 is acceleration, not a shelf replacement. A UPS is mandatory, not a perfectionist extra.

RAID buys time to replace a disk. It does not replace backup. Same-batch drives age together. Replace them one by one, mix ages, do not cheap out on power — and “everything failed at once” stays someone else’s story.