RAID Levels Explained: Which One Your NAS Should Use

RAID (redundant array of independent disks) is a way of combining two or more hard drives into one storage pool so the array keeps working, and keeps your data readable, after a drive dies. Anyone setting up a NAS for the first time needs to pick a RAID level before they load it with photos, backups, or media, because the level you choose decides how much usable space you get, how many drives can die before you lose everything, and how long a recovery takes when one does.

What Is RAID, Really

RAID is what happens when you tell your NAS to treat several physical drives as one logical volume, and to write your data across them in a pattern that survives losing one of them. Think of it like keeping a carbon copy of an important form in a second filing cabinet: if one cabinet burns down, you still have the form. RAID does that automatically, at the block level, every time a file is written. Our RAID basics explainer covers what a mirror and parity protect against; this guide focuses on which level to pick and the drive math behind it.

How It Works

Every RAID level is a different mix of three tricks: mirroring (writing the same data to two drives at once), parity (writing a mathematical checksum alongside your data so a lost drive's contents can be recalculated), and striping (splitting data across drives for speed, with no redundancy on its own). When a drive fails, the NAS rebuilds it: it either copies the mirror or recalculates the missing data from parity and writes it to a replacement drive. That rebuild is the single riskiest moment in an array's life, because every remaining drive has to be read in full while the array keeps serving your files, and any read error during that window can turn a one-drive failure into total data loss.

The RAID Levels, Compared

RAID 0 has no redundancy at all, it just stripes data for speed, so it doesn't belong in a discussion of NAS reliability except as the level to avoid. Everything else on this list trades some raw capacity for the ability to survive a failed drive.

Worked Example: 4x8TB, Four Ways

Numbers make this concrete faster than percentages do. Take a 4-bay NAS loaded with four WD Red Pro 8TB drives at about $269.99 each as of this writing, roughly $1,079.96 for the set of drives:

RAID 5 gives you the most usable space, but it also means that after one drive dies, the array runs with zero redundancy until the rebuild finishes, and rebuilding an 8TB drive means reading the full 24TB of remaining data off the other three drives. RAID 6 and SHR-2 cost you a second drive's worth of capacity, but the array can survive a second failure mid-rebuild, which is exactly the scenario that turns a bad day into a lost NAS.

Why Big Drives Make RAID 5 Rebuilds Risky

A RAID 5 rebuild has to read every surviving drive end to end, and the bigger the drives, the longer that read takes and the more data has to come back error-free. At 8TB and up, storage engineers generally stop recommending single-parity RAID 5 for exactly this reason: a real-world rebuild under load commonly runs a full day or more, and any read error on a surviving drive during that window can turn one dead drive into a lost array. RAID 6 and SHR-2 exist specifically to survive a second failure while that rebuild is happening.

Drive choice matters here too. SMR (shingled magnetic recording) drives, the kind that show up cheap and best-selling in the desktop aisle, write in overlapping tracks that have to be reshuffled for anything but simple sequential writes, and a rebuild is a sustained, mixed read/write operation. SMR drives have been documented taking dramatically longer to rebuild than CMR drives, in some cases stalling out before finishing. Every drive here, including the Seagate IronWolf 4TB and Toshiba N300 8TB, is confirmed CMR, the property that lets a rebuild finish cleanly instead of stalling.

Who Actually Needs RAID (and Who Doesn't)

You need RAID if a dead drive would mean real downtime or a real scramble: a NAS holding your only copy of family photos, a home server other people in the house depend on, or a media library you don't want to re-rip. You don't need RAID if you're using a single external drive as a portable backup target (our external hard drive guide covers that case directly), or if losing the NAS would be an inconvenience rather than a disaster. It's also not a speed upgrade: for fast local storage editing video off a laptop, a portable SSD beats any spinning-disk array on latency.

Match the level to how many bays you have, not the biggest usable-capacity number you can find:

• 2-bay NAS: RAID 1. There's no other sane choice with only two drives; mirror one to the other and accept 50% usable capacity for surviving either drive dying.
• 4-bay NAS, smaller drives (4TB and under): RAID 5 is a reasonable default; rebuild times stay manageable and you keep three drives' worth of space instead of two.
• 4-bay NAS, larger drives (8TB+): RAID 6 or SHR-2. The extra parity drive is the price of surviving a second failure during a rebuild that now takes the better part of a day.
• 6-bay or larger NAS: RAID 6 for capacity, or RAID 10 if you have bays to spare for the 50% capacity hit and want the performance.

RAID vs Synology SHR, QNAP, and TrueNAS RAID-Z

Every NAS platform wraps standard RAID in its own name, which trips up first-timers who go looking for "RAID 5" in the setup wizard and don't find it. Synology's SHR (and SHR-2) behave like RAID 5 (and RAID 6) under the hood, but calculate parity per drive size instead of requiring identical drives, so you lose less capacity mixing drive sizes across bays. QNAP's standard setup uses the conventional RAID 0/1/5/6/10 levels directly. TrueNAS, built on ZFS, calls single parity RAID-Z1 and dual parity RAID-Z2, the same tradeoffs as RAID 5 and RAID 6 with different rebuild mechanics underneath. If your NAS shows SHR or RAID-Z instead of a plain RAID number, use the table above as the translation key.

RAID Is Not a Backup

RAID protects against a dead drive. It does nothing for the other ways you lose data. A house fire, a ransomware infection, a software bug that silently corrupts a file, or you deleting the wrong folder all write themselves across every drive in the array identically, redundancy included. The standard fix is a 3-2-1 plan: your live copy on the NAS, a second copy on a separate external hard drive or portable SSD you can unplug and put in a drawer, and a third copy off-site or in the cloud. If your NAS array is the only place a file exists, RAID has bought you time during a drive failure, not safety from everything else.

What to Look For When You Build the Array

• CMR, not SMR. Confirm the recording method on the spec sheet or box before you buy; SMR drives rebuild poorly and some vendors have shipped SMR silently in non-NAS product lines.
• Matched or close RPM class across the array. Mixing a 5,400 RPM drive with a 7,200 RPM drive works, but you'll get the slower drive's performance for the whole array.
• Workload rating (TB/year) that matches how busy the NAS will be. A 24/7 media server pushing constant reads wants a higher-rated drive than an occasional backup target.
• Warranty length as a signal, not just a safety net. Longer warranties generally track the manufacturer's own confidence in a drive's failure rate.
• A second vendor for at least one bay. Running every bay from the same manufacturing batch means a bad batch can take out multiple drives close together; mixing brands avoids that correlated risk.

The Best NAS Drives You Can Buy Right Now

WD Red Plus 4TB NAS Hard Drive (WD40EFPX)

At about $179.99 as of this writing, this is the drive to start a first RAID 1 mirror with. It's confirmed CMR, rated for 24/7 operation across up to 8 bays, and owners running it around the clock in RAID report it stays quiet and reports healthy SMART values for years. The recurring complaint is a subset of units arriving with a loud clicking or seek noise out of the box, usually resolved through a warranty exchange rather than something you can fix, so test a new drive for a few days before you trust it with an array.

Seagate IronWolf 4TB NAS Hard Drive (ST4000VN008)

Priced about the same as the Red Plus, around $179.99 as of this writing, which is exactly why it's the standard second-vendor pick: pairing it with WD drives in the same array avoids putting every bay on one manufacturer's firmware batch. Its AgileArray firmware and rotational vibration sensors are built specifically for multi-bay vibration. The catch is the actual spindle speed is 5,900 RPM, not the 7,200 RPM some retailer listings imply, so don't expect a performance jump over the Red Plus.

WD Red Pro 8TB NAS Hard Drive (WD8005FFBX)

This is the drive behind the worked example above: about $269.99, 7,200 RPM, and a 300TB/year workload rating that's 67% higher than the budget-tier drives here, which matters once you're running four-plus drives in RAID 5 or RAID 6 with real daily traffic. The 5-year warranty is two years longer than the entry-tier drives in this lineup. A small but recurring share of buyers report early-life failures despite that warranty, which is the whole argument for choosing RAID 6 over RAID 5 once you're building with drives this size.

Toshiba N300 8TB NAS Hard Drive (HDWG480XZSTA)

At about $229.99 it undercuts the Red Pro by roughly $40 while still spinning at 7,200 RPM with confirmed CMR, which is why it's worth considering as a third brand in a mixed-vendor array. The workload rating caps out at 180TB/year, the same budget-tier number as the 5,400 RPM Red Plus despite the faster spindle, and warranty coverage trails the Red Pro at 3 years versus 5. The most-cited complaint is early-weeks failures for a meaningful share of buyers; drives that get past the first month tend to run trouble-free for years, which is the case for burning in any new drive for a few days before you commit it to a rebuild-critical array.

Bottom Line

Pick the RAID level for the bays you actually have, not the one with the biggest usable-capacity number. A 2-bay NAS means RAID 1 and a pair of WD Red Plus 4TB drives. A 4-bay NAS with smaller drives can run RAID 5, but once you're buying WD Red Pro 8TB or Toshiba N300 8TB drives, step up to RAID 6 or SHR-2 so a rebuild doesn't leave you exposed for a full day with no redundancy left. Whichever level you choose, keep a real backup outside the array; RAID has never once protected anyone from deleting the wrong folder.