If you’re building a home NAS — whether on TrueNAS SCALE, Unraid, or a Synology unit — understanding RAID is essential. RAID (Redundant Array of Independent Disks) determines how your drives work together: how fast data reads and writes, how many drives you can lose before losing data, and how much usable storage you actually get. This guide explains every major RAID level with real-world numbers and tells you which one to pick for your setup.
What RAID Is (and What It Isn’t)
RAID is not a backup. This cannot be overstated. RAID protects against drive failure, but it does nothing to protect against ransomware, accidental deletion, fire, theft, or a corrupted filesystem taking down the whole array. Always follow the 3-2-1 backup rule: three copies, two media types, one offsite.
With that established, RAID is still extremely valuable for keeping your NAS online when a drive dies — which, with spinning hard drives running 24/7, is a matter of when, not if.
RAID Levels Explained
RAID 0 — Striping (No Redundancy)
RAID 0 splits data evenly across two or more drives. If you have two 4 TB drives, you get 8 TB of usable space and roughly double the sequential read/write speed. Sounds great — until one drive fails and you lose everything on both drives.
Best for: Scratch storage, video editing cache, or non-critical temp files where speed matters more than data safety. Never use RAID 0 for anything you care about.
| Drives | Usable Space | Fault Tolerance |
|---|---|---|
| 2x 4TB | 8 TB | 0 drives |
| 4x 4TB | 16 TB | 0 drives |
RAID 1 — Mirroring
RAID 1 writes identical data to two drives simultaneously. You lose half your raw capacity, but you can survive the failure of one drive with no data loss. Read speeds can be improved (some controllers read from both drives), but write speed is limited to one drive.
Best for: OS drives, critical databases, or any two-drive NAS where you want the simplest possible redundancy. Synology two-bay units and most entry-level home NAS boxes default to RAID 1.
| Drives | Usable Space | Fault Tolerance |
|---|---|---|
| 2x 4TB | 4 TB | 1 drive |
| 2x 8TB | 8 TB | 1 drive |
RAID 5 — Striping with Parity
RAID 5 requires at least three drives. Data and parity information are striped across all drives, so you lose one drive’s worth of capacity to parity. With four 4 TB drives you get 12 TB usable. You can lose any one drive and rebuild from the parity data on the remaining drives.
The catch: during a rebuild, the surviving drives are under heavy stress for hours or days. With large modern drives (16–20 TB), a RAID 5 rebuild can take 24–72 hours. If a second drive fails or develops an uncorrectable read error (URE) during that window, you lose everything.
Best for: Three- to four-drive arrays with drives 8 TB or smaller. WD Red Plus and Seagate IronWolf are purpose-built for NAS RAID use.
| Drives | Usable Space | Fault Tolerance |
|---|---|---|
| 3x 4TB | 8 TB | 1 drive |
| 4x 6TB | 18 TB | 1 drive |
| 5x 4TB | 16 TB | 1 drive |
RAID 6 — Striping with Double Parity
RAID 6 adds a second parity block, meaning you can lose any two drives simultaneously without data loss. You sacrifice two drives’ worth of capacity. Rebuilds are slower than RAID 5 but you have a safety net if a second drive fails during the process.
Best for: Arrays with five or more large drives (12 TB+), or any production NAS where downtime is unacceptable. If you’re running 16–20 TB drives like the Seagate Exos X20, RAID 6 is strongly recommended over RAID 5.
| Drives | Usable Space | Fault Tolerance |
|---|---|---|
| 4x 6TB | 12 TB | 2 drives |
| 6x 8TB | 32 TB | 2 drives |
| 8x 4TB | 24 TB | 2 drives |
RAID 10 — Striped Mirrors
RAID 10 (sometimes called RAID 1+0) combines mirroring and striping. Drives are paired into mirrored sets, and those sets are striped together. You need a minimum of four drives and lose half your capacity, but you get excellent read/write performance and can survive multiple drive failures — as long as both drives in the same mirror pair don’t fail simultaneously.
Best for: High-performance workloads where you need both speed and redundancy. Home Plex servers with heavy transcoding, small business file servers, or virtualization hosts benefit most. Rebuilds are also much faster than RAID 5/6 since you’re copying from a mirror rather than recalculating parity.
| Drives | Usable Space | Fault Tolerance |
|---|---|---|
| 4x 4TB | 8 TB | 1–2 drives |
| 6x 6TB | 18 TB | 1–3 drives |
| 8x 4TB | 16 TB | 1–4 drives |
Choosing the Right RAID Level
| Use Case | Recommended RAID | Why |
|---|---|---|
| 2-drive home NAS | RAID 1 | Simplest redundancy |
| Media server (3–4 drives, small HDDs) | RAID 5 | Good balance of space and protection |
| Media server (5+ drives, large HDDs) | RAID 6 | Survives 2-drive failure |
| High-performance file server | RAID 10 | Speed + redundancy |
| Temporary scratch space | RAID 0 | Maximum speed, no care for data |
ZFS vs. Traditional RAID
If you’re running TrueNAS SCALE or Proxmox with ZFS, you’re not technically using hardware RAID — you’re using ZFS RAIDZ. The concepts map closely:
- RAIDZ1 ≈ RAID 5 (one parity drive)
- RAIDZ2 ≈ RAID 6 (two parity drives)
- RAIDZ3 = three parity drives (no traditional equivalent)
ZFS adds data integrity checksums that hardware RAID lacks. It will detect and correct silent data corruption (bitrot) automatically, which spinning drives are prone to over years of operation. For a home NAS, TrueNAS SCALE with RAIDZ2 on a four- to six-drive array is the gold standard.
Setting Up RAID on TrueNAS SCALE
- Install TrueNAS SCALE on a dedicated SSD (at least 16 GB, separate from your data drives).
- Navigate to Storage > Create Pool.
- Select your drives and choose your VDEV layout (RAIDZ1, RAIDZ2, Mirror, etc.).
- Assign the pool a name and click Create.
- Create datasets within the pool for organized storage (media, backups, documents).
For Synology NAS owners, the Storage Manager walks you through the same process with a graphical wizard. Synology’s SHR (Synology Hybrid RAID) is a proprietary variant that allows mixing drive sizes efficiently — useful if you’re expanding an older array with larger drives over time.
Recommended NAS Drives
- WD Red Plus (CMR, 2–14 TB): Best for RAID arrays in home NAS. Avoids SMR pitfalls.
- Seagate IronWolf (1–20 TB): Purpose-built for NAS, includes IronWolf Health Management.
- Seagate Exos X20 (20 TB): Enterprise-class for larger arrays. Loud but extremely reliable.
- Toshiba N300 (4–16 TB): Good alternative at competitive pricing.
Avoid SMR (Shingled Magnetic Recording) drives in RAID. Drives like the WD Red (non-Plus) use SMR technology that performs catastrophically poorly during RAID rebuilds. Stick to CMR drives for any NAS application.
Final Thoughts
For most home users building a first NAS, RAID 1 on two drives or RAIDZ1/RAID 5 on three to four drives covers the majority of needs. As your storage grows past 10 TB total and your drives grow past 8 TB each, step up to RAIDZ2 or RAID 6 for the extra safety margin during rebuilds. And no matter which level you choose — back up your data. RAID keeps you online when a drive fails; backups keep your data safe when everything else goes wrong.