RAID 1 vs RAID 5 vs RAID 6 for a Home NAS

Quick Summary (TL;DR):
RAID 1 is the simplest choice for a two-bay home NAS: two equal drives provide roughly one drive’s usable capacity and the array can survive one drive failure. RAID 5 starts at three drives and sacrifices one drive’s worth of capacity to parity; it offers the best capacity efficiency when you accept one-drive fault tolerance. RAID 6 starts at four drives and sacrifices two drives’ worth of capacity, but can survive any two drive failures. The right choice depends heavily on bay count. Four-bay RAID 6 uses only 50% of raw capacity, while six-bay RAID 6 uses about 67% and eight-bay RAID 6 about 75%. For a normal two-bay family NAS, RAID 1 is straightforward. For a four-bay NAS, RAID 5 is often the capacity-efficient option, while RAID 6 makes sense when the data is important enough that dual-drive tolerance outweighs losing half the raw capacity. For six- and eight-bay arrays—especially with large 16–24TB drives—RAID 6 becomes much easier to justify because rebuilds involve more data and the parity overhead is proportionally smaller. Most importantly, RAID is not a backup: it protects availability after disk failure, not against deletion, ransomware, filesystem corruption, theft, fire or a failed NAS. Keep an independent backup regardless of RAID level.

RAID 1 vs RAID 5 vs RAID 6 at a Glance

RAID level	Typical minimum drives	Drive failures tolerated	Usable capacity with equal drives	Best fit
RAID 1	2	1	≈ 1 drive	2-bay home NAS
RAID 5	3	1	≈ (N − 1) drives	Capacity-efficient 3–5 bay NAS
RAID 6	4	2	≈ (N − 2) drives	Important data / 6–8+ bay NAS

What RAID Actually Protects You From

RAID combines multiple physical drives so that a NAS can continue operating after selected disk failures. That protects against one very specific event: a physical member drive fails.

Depending on RAID level, the array remains available in a degraded state while the failed drive is replaced and rebuilt.

What RAID Does Not Protect You From

Deleting the wrong folder.
Ransomware encrypting the files.
A software bug corrupting data.
Filesystem corruption.
NAS theft.
Fire or flood.
Electrical damage affecting the entire enclosure.
An administrator mistake.
A bad application synchronising unwanted changes everywhere.

RAID improves availability. Backup provides independent recoverability. Those are different jobs.

RAID 1: Simple Mirroring

In the common two-drive RAID 1 configuration, the same data is written to both disks.

Drive 1: A B C D
Drive 2: A B C D

If one drive fails, the second still contains a complete copy of the array data. After installing a replacement disk, the NAS copies the surviving data back to the new member to restore redundancy.

RAID 1 Capacity

2 × 12TB RAID 1
raw = 24TB
usable ≈ 12TB before filesystem/system overhead

You effectively pay for two drives and receive one drive’s capacity.

Why RAID 1 Works So Well in a 2-Bay NAS

Simple to understand.
One-drive fault tolerance.
Easy replacement process.
No parity calculation.
Excellent fit for family backups, photos and media.

For a two-bay NAS, there is little reason to overcomplicate the design if RAID 1 provides enough capacity.

RAID 1’s Main Weakness Is Capacity Efficiency

RAID 1 always pays a large capacity penalty because the second drive mirrors the first. With two 20TB drives you buy 40TB raw but receive about 20TB decimal usable before overhead.

If you need 40–80TB usable, a larger multi-bay NAS with parity RAID can be more economical.

RAID 5: One Drive’s Worth of Parity

RAID 5 stripes data and parity across all member drives. No single drive is permanently “the parity drive”; parity blocks are distributed across the array.

Drive 1: Data A | Data D | Parity
Drive 2: Data B | Parity | Data E
Drive 3: Parity | Data C | Data F

If one drive fails, the missing information can be reconstructed from the remaining data and parity.

RAID 5 Capacity Formula

usable capacity
≈ (number of drives − 1)
× smallest drive size

Array	Raw capacity	Approx. RAID 5 usable	Efficiency
3 × 12TB	36TB	24TB	67%
4 × 12TB	48TB	36TB	75%
6 × 12TB	72TB	60TB	83%
8 × 12TB	96TB	84TB	88%

Actual available capacity is lower after binary/decimal conversion, system partitions and filesystem overhead.

Why RAID 5 Is Attractive

Only one drive’s worth of capacity is lost to parity.
Read throughput can scale across several drives.
Works well for large sequential files.
Good capacity efficiency in four- and five-bay systems.
Can survive one drive failure.

RAID 5’s Weakness: The Array Is Exposed During Rebuild

After one drive fails, RAID 5 has no remaining disk-failure tolerance until the replacement drive has been rebuilt. If a second member drive fails before the rebuild completes, the RAID group is lost.

This does not mean “RAID 5 is unsafe”. It means the degraded/rebuild period deserves serious consideration as drives and arrays become larger.

Large Drives Make Rebuild Exposure More Important

A modern 20TB or 24TB drive can contain an enormous amount of data. Rebuilding a large array requires sustained reads from the surviving members and sustained writes to the replacement.

Actual rebuild time depends on drive speed, NAS CPU/controller, background workloads, RAID implementation, drive health and how much data is stored. Do not use simplistic rules such as “a 20TB drive takes exactly X hours”.

RAID 6: Two Independent Parity Sets

RAID 6 extends parity RAID so the array can survive any two member-drive failures. It costs the equivalent capacity of two drives instead of one.

usable capacity
≈ (number of drives − 2)
× smallest drive size

RAID 6 Capacity Examples

Array	Raw capacity	Approx. RAID 6 usable	Efficiency
4 × 12TB	48TB	24TB	50%
6 × 12TB	72TB	48TB	67%
8 × 12TB	96TB	72TB	75%

This explains why RAID 6 feels expensive in a four-bay NAS but becomes much more attractive in six- and eight-bay systems.

RAID 6’s Main Advantage Is the Rebuild Safety Margin

After one drive fails, a RAID 6 array still retains one additional drive of failure tolerance. That is valuable because drive replacement and rebuild are not instantaneous.

In a large array holding important data, that extra margin can be worth far more than one additional drive’s capacity.

RAID 6’s Main Disadvantage Is Write Overhead and Capacity Cost

RAID 6 calculates/stores two parity values, so writes can involve more parity work than RAID 5. On a modern home NAS, the performance difference depends heavily on CPU, drive count, filesystem, write pattern, cache and network speed.

For ordinary 1GbE or 2.5GbE home backups, RAID 6 parity overhead may be far less important than the extra fault tolerance.

4-Bay NAS: RAID 5 vs RAID 6

4 × 16TB example	RAID 5	RAID 6
Raw capacity	64TB	64TB
Approx. usable	48TB	32TB
Drive failures tolerated	1	2
Raw efficiency	75%	50%

RAID 5 gives 50% more usable capacity in this example. RAID 6 gives one additional drive of failure tolerance. There is no universal winner.

Choose RAID 5 in Four Bays When

Capacity efficiency is important.
You maintain a proper independent backup.
The NAS is not business-critical.
You can replace a failed drive quickly.
Your data can tolerate some downtime if a worst-case second failure occurs.

Choose RAID 6 in Four Bays When

Availability is more important than capacity.
The data is difficult to restore quickly.
The NAS is in a remote location.
You use very large HDDs.
You want the array to remain one-drive-failure tolerant after the first failure.

6-Bay NAS: RAID 6 Becomes Much Easier to Justify

6 × 16TB	RAID 5	RAID 6
Raw capacity	96TB	96TB
Approx. usable	80TB	64TB
Drive failures tolerated	1	2
Raw efficiency	83%	67%

You give up 16TB of usable capacity versus RAID 5 but gain dual-drive fault tolerance. For a serious six-bay home lab, RAID 6 is often the more comfortable choice.

8-Bay NAS: RAID 6 Is Highly Capacity-Efficient

8 × 16TB	RAID 5	RAID 6
Raw capacity	128TB	128TB
Approx. usable	112TB	96TB
Drive failures tolerated	1	2
Raw efficiency	88%	75%

At eight bays, sacrificing two drives to parity still leaves 75% of the raw array capacity usable. This is one reason RAID 6 is common in larger arrays.

Why Drive Size and Bay Count Must Be Chosen Together

RAID level cannot be separated from drive economics. For example, a four-bay RAID 6 with 20TB drives gives about 40TB usable, while six 12TB drives in RAID 6 give about 48TB usable.

Which is cheaper depends on current drive and enclosure prices. Our live-cost article does the maths: 8TB vs 12TB vs 16TB vs 20TB NAS Drives: Cost per Usable Terabyte.

RAID Uses the Smallest Drive as the Capacity Baseline

With standard RAID and mixed-size members, usable capacity is normally limited by the smallest drive in the group.

12TB + 12TB + 20TB + 20TB
standard RAID calculation
treats each member as roughly 12TB capacity

The extra space on the larger members may be unusable until the smaller drives are replaced or the platform provides a flexible RAID technology. Synology SHR and TerraMaster TRAID approach mixed-size expansion differently; that is covered in the next article rather than here.

Do Not Confuse RAID 1 with “Two Backups”

Both mirrored drives contain the same current filesystem state.

delete file
→ RAID writes deletion to both mirrors

If ransomware encrypts a share, both copies now contain encrypted data. RAID 1 is redundancy, not versioned backup.

Snapshots Help, but They Still Are Not an Independent Backup

Filesystem snapshots are extremely useful for recovering deleted or changed files. But snapshots normally live on the same NAS/storage pool.

A catastrophic enclosure failure, theft, fire or pool corruption can therefore affect both current data and snapshots. Use snapshots and independent backups.

A Good Home NAS Protection Stack

RAID
→ protects against selected drive failures

snapshots
→ protects against many accidental changes/deletions

local independent backup
→ protects against NAS/pool failure

off-site backup
→ protects against site-level loss

The upcoming backup architecture guide puts these layers together: A 3-2-1 NAS Backup Plan: USB Drive, Second NAS and Cloud.

Rebuilds Are When Redundancy Is Most Valuable

A RAID array is not “fixed” the moment you replace a failed disk. The new member has to be rebuilt/resynchronised.

All surviving drives are heavily read.
Performance may be reduced.
RAID 5 has no remaining disk-failure tolerance.
RAID 6 still tolerates one additional drive failure.

Keep a Replacement Strategy

A RAID 6 array provides more time, but a failed drive should still be replaced promptly.

A known-compatible cold spare nearby.
Fast retailer/vendor replacement access.
SMART/health alerts.
Email/push notifications.
A UPS to avoid power interruption during rebuild.

Hot Spare vs Cold Spare

A hot spare sits installed and can be used automatically when a member fails. A cold spare is stored separately and manually installed when needed.

For a small home NAS, a cold spare can be more capacity-efficient because the bay remains available for normal storage. For a remote or business-critical NAS, automatic hot-spare response can be valuable.

Should a 2-Bay NAS Use RAID 1?

Usually yes if both bays are part of the same primary storage pool. Alternatives such as two independent disks can provide more raw capacity or separate backup roles, but they do not provide automatic array continuity after a single drive failure.

For most families that want one simple shared volume, RAID 1 is the obvious layout.

Should a 4-Bay NAS Use RAID 5?

RAID 5 is still a sensible four-bay home NAS choice when you need the extra capacity, have good backups, the NAS is not mission-critical, and you can replace failed disks quickly.

It is not automatically obsolete simply because modern drives are large.

Should a 4-Bay NAS Use RAID 6?

RAID 6 makes sense when availability and dual-drive tolerance matter more than raw capacity. The 50% capacity efficiency is expensive, however. If you need both high capacity and dual parity, moving to a six-bay NAS may be financially smarter.

See 2-Bay vs 4-Bay vs 6-Bay NAS: Which Size Should You Buy?.

Should a 6- or 8-Bay NAS Use RAID 6?

For important data, RAID 6 becomes increasingly attractive as the array grows. The two-drive parity cost is spread across more data drives, while the extra failure tolerance becomes more valuable during long rebuilds.

That does not mean every eight-bay home NAS needs RAID 6, but it is where RAID 6’s trade-off is easiest to justify.

RAID Performance: Do Not Overgeneralise

RAID 5/6 can improve sequential read throughput because data is distributed across several drives. Writes require parity work.

Real performance depends on NAS CPU, number/type of drives, filesystem, cache, network speed, file size and concurrent workloads. Do not choose RAID 5 over RAID 6 based on an assumed universal MB/s difference.

1GbE and 2.5GbE Often Hide RAID Performance Differences

A four- or six-drive HDD RAID can already exceed Gigabit Ethernet sequentially. If your clients are limited to 1GbE, both RAID 5 and RAID 6 may feel equally fast for large file reads because the network is the bottleneck.

At 10GbE, array design becomes much more visible.

RAID Does Not Fix a Bad Drive Choice

Verify CMR vs SMR.
Verify NAS compatibility.
Check workload rating.
Check warranty.
Check drive health before adding a member.

RAID cannot make an unsuitable or failing disk behave like a proper NAS drive.

A Practical RAID Decision Matrix

Your situation	Likely best direction	Reason
2-bay family NAS	RAID 1	Simple one-drive redundancy
3-bay NAS	RAID 5	Only practical parity option among these
4-bay, capacity-first	RAID 5	75% raw efficiency
4-bay, availability-first	RAID 6	Two-drive fault tolerance
6-bay important data	RAID 6	Good 67% raw efficiency + dual parity
8-bay important data	RAID 6	75% raw efficiency + dual parity
Large drives + weak backup	Fix backup first	RAID cannot replace recovery copies
Mixed drive sizes	Check SHR/TRAID/flexible RAID	Standard RAID wastes larger-drive excess capacity

RAID 1 vs RAID 5 vs RAID 6: The Bottom Line

RAID 1 is the natural two-bay choice. It is simple, predictable and provides one-drive fault tolerance at the cost of half the raw capacity.

RAID 5 is still a rational home NAS layout. In four bays it provides 75% raw capacity efficiency and one-drive tolerance, making it attractive when independent backups are strong and capacity matters.

RAID 6 becomes more compelling as arrays grow. Four-bay RAID 6 is expensive at 50% efficiency, but six-bay RAID 6 reaches about 67% and eight-bay RAID 6 reaches 75% while retaining two-drive failure tolerance.

Large drives strengthen the case for thinking about rebuild exposure. More data must be reconstructed, and RAID 6 retains one additional disk-failure margin after the first failure.

Most importantly, choose RAID for availability—not as a replacement for backup. A well-designed home NAS uses RAID, snapshots and independent backup together.