RAID Levels Explained for Small Business NAS: RAID 1 vs 5 vs 6 vs 10

You bought a four-bay NAS for the office, dropped in four drives, and now the setup wizard is asking you to choose a RAID level. RAID 1? RAID 5? SHR? The wizard helpfully explains nothing about the tradeoffs, and the choice you make here is hard to change later without copying everything off and rebuilding. This guide cuts through it: what each RAID level actually does, how much usable space you give up, and which one fits a small business with two, four, or more drives.

What RAID Does and Doesn’t Do

RAID — Redundant Array of Independent Disks — combines multiple physical drives so that the failure of one (or more) does not lose your data and, in most cases, does not even take the system offline. The key word is availability. RAID keeps your shared files reachable when a drive dies, and lets you swap the dead drive and rebuild without downtime.

Here is the part every guide must say and many bury: RAID is not a backup. RAID protects against drive failure. It does nothing against accidental deletion, ransomware, a corrupted file, a fire, theft, or a power surge that takes out the whole unit. If someone deletes the wrong folder, RAID faithfully deletes it from every drive instantly. You still need a separate backup — ideally following the 3-2-1 rule: three copies, two media types, one off-site. RAID is the foundation; backup is the insurance. Build both.

With that settled, here are the levels you will actually be offered.

RAID 1: Mirroring (2 Drives)

RAID 1 writes identical copies of everything to two drives. If one fails, the other has a complete copy and you keep working. It is the simplest form of redundancy.

• Usable capacity: 50%. Two 8 TB drives give you 8 TB usable.
• Survives: one drive failure.
• Read/write performance: reads can be slightly faster (two drives to pull from); writes are about the speed of a single drive.
• Rebuild: fast and low-risk, because it is a straight copy from the surviving drive.

RAID 1 is the right answer for a 2-bay NAS, which is the most common small-business unit. You sacrifice half your raw capacity, but you get dead-simple, robust protection and trivially easy rebuilds. For a small office storing documents, accounting files, and shared folders on a 2-bay box, RAID 1 plus a real backup is hard to beat.

RAID 5: Striping With Single Parity (3+ Drives)

RAID 5 spreads data across all drives and dedicates the equivalent of one drive’s worth of space to parity — error-correcting information that lets the array reconstruct a failed drive’s contents. You lose one drive’s capacity total, regardless of how many drives you have.

• Usable capacity: (N − 1) drives. Four 8 TB drives give 24 TB usable.
• Survives: one drive failure.
• Performance: good read speeds; writes carry a parity-calculation penalty.
• Rebuild: this is the catch. See below.

RAID 5 is attractive because the capacity cost is low — you only “lose” one drive no matter how many you have. For a 4-bay NAS it is a popular default. But RAID 5 has a well-known weakness during rebuilds. When a drive fails and you replace it, the array must read every block on every remaining drive to reconstruct the missing data. That is a heavy, hours-to-days-long stress test on aging drives, exactly when they are most likely to fail. If a second drive dies during the rebuild, the entire array is lost. With today’s large drives (12 TB and up), rebuild times stretch long enough that this risk is real, which is why many people now favor RAID 6 for large arrays.

RAID 6: Striping With Double Parity (4+ Drives)

RAID 6 is RAID 5 with a second, independent parity calculation. It costs you two drives’ worth of capacity but survives two simultaneous drive failures.

• Usable capacity: (N − 2) drives. Four 8 TB drives give 16 TB usable; six give 32 TB.
• Survives: two drive failures.
• Performance: slightly slower writes than RAID 5 due to the extra parity.
• Rebuild: the big win. You can lose one drive, start rebuilding, and survive a second failure during that rebuild — directly addressing RAID 5’s worst-case scenario.

For any array of large drives, or any array where the data matters and downtime is expensive, RAID 6 is the conservative, recommended choice once you have four or more bays. You give up an extra drive’s capacity for meaningful protection against the most dangerous failure window. On a 4-bay unit, RAID 6 leaves you only two drives of usable space, which feels stingy; its value grows as bay count rises, making it ideal for 6-, 8-, or 12-bay units.

RAID 10: Mirroring Plus Striping (4+ Drives)

RAID 10 (also written 1+0) combines mirroring and striping. Drives are paired into mirrors, and data is striped across those pairs. With four drives, you have two mirrored pairs.

• Usable capacity: 50%. Four 8 TB drives give 16 TB usable.
• Survives: at least one drive failure, and up to one per mirror pair — so potentially two failures, as long as they are not both in the same pair.
• Performance: the fastest of the group for both reads and writes, with no parity calculation overhead.
• Rebuild: fast and low-stress, because rebuilding only copies from the surviving mirror, not the whole array.

RAID 10 is the choice when performance matters — think hosting a database, virtual machines, or video editing scratch space off the NAS. It costs you half your capacity like RAID 1, but the fast, low-risk rebuilds and strong performance make it the pick for demanding workloads. For pure file storage, that performance is usually overkill and RAID 6 gives more usable space.

SHR: Synology’s Flexible Option

If you are on a Synology NAS, you will also see SHR (Synology Hybrid RAID). It is not a different math — under the hood it is RAID 1, 5, or 6 — but it lets you mix drives of different sizes and use more of their capacity than standard RAID allows, and it makes expanding the array later much easier. For a small business that buys drives over time and does not want to think hard about it, SHR (or SHR-2 for double-drive protection) is a sensible default.

Quick Recommendations by Drive Count

• 2 bays: RAID 1. Simple, robust, easy rebuilds.
• 4 bays, capacity-focused: RAID 6 if the data matters; RAID 5 only if you accept the rebuild risk and keep solid backups.
• 4 bays, performance-focused: RAID 10.
• 6+ bays: RAID 6 (or SHR-2 on Synology) as the default; RAID 10 if you need speed.
• Any setup: pair it with a real, separate backup. Always.

Pick the level that matches your bay count and how much downtime would cost you, size your usable capacity with the formulas above, and remember the one rule that survives every RAID debate: the array keeps you running, but the backup is what actually saves you.