What RAID Does in a Home NAS
A network-attached storage (NAS) box is more than a few hard drives in a case. What turns a stack of disks into reliable storage is RAID — a Redundant Array of Independent Disks. RAID decides how your data is spread across the drives inside the enclosure, and that decision controls two things at once: how much usable space you end up with, and how many dead drives you can survive before your files are gone.
Many first-time NAS buyers make the same mistake. They buy four 4 TB drives, expect 16 TB of space, and are surprised to discover the usable number is considerably smaller. The gap is not a defect. It is RAID doing its job — trading some capacity for protection. Understanding that trade-off before you buy drives is the difference between a storage setup you can trust and one that quietly disappoints you.
RAID 0 vs RAID 1: Speed Versus Safety
RAID 0, also called striping, writes data in pieces across every drive at once. Because all drives work in parallel, it is fast — and because there is no redundancy data, you get to use the full combined capacity of every disk. The catch is brutal: if any single drive fails, the whole array fails, and recovery is effectively impossible. RAID 0 is fine for temporary scratch space or video editing caches where the files exist somewhere else too. It is never where precious memories should live.
RAID 1 is the opposite idea. Called mirroring, it writes every byte to two drives at once, so each disk is an exact copy of the other. If one drive dies, the other carries on without a hiccup. The price is capacity: a pair of 4 TB drives gives you just 4 TB of usable space. For a two-bay NAS holding family photos, documents and phone backups, RAID 1 is the sensible default. It is simple, it rebuilds fast, and it answers the question "what if a drive dies?" with confidence.
RAID 5: The Sweet Spot for a Four-Bay NAS
RAID 5 is the most popular choice for home and small-office NAS units with three or more drive bays. It stripes data across all drives like RAID 0, but it also distributes parity information — mathematical recovery data — across the array. If one drive fails, the missing pieces are rebuilt from the parity. You get most of your raw capacity back: with N drives, the usable space equals N minus one drives' worth.
RAID 5 usable capacity = (number of drives − 1) × size of the smallest drive
A concrete example: four 4 TB drives in RAID 5 yield 12 TB of usable space — one drive's worth is reserved for parity. There is a modest write-performance cost, because every write must also update the parity data, but for media storage, file sharing and backups it is rarely noticeable. The rebuild after replacing a failed drive can take a day or more with large modern disks, during which a second failure would be fatal — which is why backups still matter even with RAID 5.
RAID 6 and RAID 10: When You Need More Protection
RAID 6 works like RAID 5 but keeps two independent sets of parity instead of one. It survives two simultaneous drive failures, which matters more than it sounds: when one large drive fails, the long rebuild stresses the remaining drives, and a second failure during that window is the classic way RAID 5 arrays die. The cost is another drive's worth of capacity — usable space equals N minus two drives — plus a heavier write penalty. RAID 6 is the right call for arrays of six or more bays, drives of 8 TB and larger, or data you genuinely cannot replace.
RAID 10 takes a different approach. It mirrors drives in pairs (like RAID 1), then stripes across the pairs (like RAID 0). It needs an even number of drives, gives you half the raw capacity, and delivers the best of both worlds: fast rebuilds, strong performance and the ability to survive one failure in every mirrored pair. It is excellent for virtual machines and databases, though for ordinary file storage at home it is usually more capacity than the speed justifies.
How the RAID Levels Compare
The table below summarizes what each common level gives you. Here, "d" is the size of the smallest drive in the array, and N is the number of drives.
| RAID level | Usable capacity | Drive failures tolerated | Minimum drives | Best for |
|---|---|---|---|---|
| RAID 0 | N × d | None | 2 | Temporary scratch data only |
| RAID 1 | 1 × d | None→ see below | 2 | Two-bay NAS, maximum simplicity |
| RAID 5 | (N − 1) × d | 1 | 3 | Four-bay home NAS, best balance |
| RAID 6 | (N − 2) × d | 2 | 4 | Large arrays, irreplaceable data |
| RAID 10 | (N ÷ 2) × d | 1 per mirrored pair | 4 | Speed plus redundancy |
Note on the table: RAID 1 tolerates 1 drive failure (each disk mirrors the other).
How to Calculate Usable Capacity Step by Step
Before buying drives or configuring an array, work through these four steps so the number on the box matches reality:
- Find the smallest drive. In a standard RAID array, every drive is treated as if it were the size of the smallest member. A 4 TB drive mixed with three 8 TB drives wastes 4 TB per large disk — 12 TB in total. Match your drives.
- Count your bays. The number of drives determines which RAID levels are even available. Two bays means RAID 0 or 1; three or more opens up RAID 5 and beyond.
- Apply the formula. Pick your RAID level from the table above and multiply. Four 6 TB drives in RAID 5 give (4 − 1) × 6 = 18 TB of raw usable capacity.
- Subtract filesystem overhead. The file system itself — Btrfs, ext4 or ZFS — needs room for its own metadata. Reserve roughly 5–10 percent on top. That 18 TB example lands around 16–17 TB of real, usable space once the array is formatted and running.
Why Your Actual Space Is Even Smaller
Two quieter forces shave a little more off the number. First, drive makers define a terabyte as 1,000 gigabytes while your NAS reports in tebibytes of 1,024 — so a "4 TB" drive shows up as roughly 3.64 TiB before anything else happens. Second, some NAS systems reserve a slice of every volume for snapshots and system use. Neither is a fault in your hardware; they are just the difference between marketing math and storage math. The practical rule is simple: buy about 20 percent more raw capacity than you think you need, and you will rarely be caught short.
Choosing the Right RAID Level for Your Needs
- Two-bay NAS, family files: RAID 1. Simple, safe, and fast to rebuild.
- Four-bay NAS, general home use: RAID 5. The best capacity-for-protection balance most households will ever need.
- Irreplaceable data or drives of 8 TB and up: RAID 6. The second parity disk is cheap insurance during long rebuilds.
- Mixed drive sizes: consider a hybrid scheme such as Synology Hybrid RAID (SHR), which wastes less space than classic RAID when drives differ in size.
- Virtual machines or heavy multitasking: RAID 10, if your NAS has the bays and your budget tolerates halved capacity.
Whichever you choose, remember what RAID does not do. It protects against drive failure — not against accidental deletion, ransomware, fire, theft or a lightning strike on the NAS itself. A RAID array is availability insurance, not a backup. The classic 3-2-1 rule still applies: three copies of important data, on two different kinds of media, with one copy off site.
Plan It Before You Buy
Running the numbers by hand for every drive combination is tedious, especially when you are comparing two-bay versus four-bay enclosures or wondering whether that extra parity disk is worth the money. A planner does the arithmetic for you: enter your drive sizes and count, and it shows the usable capacity under each RAID level side by side.
https://veltrixnews.online/tools/nas-raid-storage-planner
Try the NAS RAID Storage Planner above with your own drive list before you order hardware — it takes thirty seconds and can easily save you from buying the wrong drives.
Frequently Asked Questions
What RAID level is best for a home NAS?
For most households, RAID 5 in a four-bay NAS is the sweet spot: it survives one drive failure while keeping most of your capacity. A two-bay NAS should use RAID 1. Step up to RAID 6 only if your data is irreplaceable or you are using very large drives, where rebuilds take long enough that a second failure becomes a real risk.
Can I change RAID levels later without losing data?
Often, yes. Most modern NAS operating systems support online RAID migration — for example, moving from RAID 1 to RAID 5 when you add drives, or from RAID 5 to RAID 6. The process can take many hours and stresses the drives, so take a full backup first. You cannot migrate out of RAID 0 without wiping it, since there is no redundancy to convert from.
Does RAID replace backups?
No. RAID keeps your storage running when a drive dies, but it cannot help with deleted files, ransomware, theft, fire or a failed NAS controller. Every RAID array should still be backed up — ideally following the 3-2-1 rule with an off-site copy — because RAID is about uptime, not about keeping a second copy of your data.