Introduction

So, you’re thinking about setting up a (7-Bay)-NAS in your homelab—awesome! Maybe you’re eyeing the UNAS-Pro, just like I am. Naturally, you’re considering different RAID options, and RAID 5 seems appealing because it maximizes storage. But hold up! RAID 5 is a ticking time bomb when using large hard drives.

The culprit? Unrecoverable Read Errors (UREs)—a small but deadly issue that can turn a simple drive failure into total data loss. In this post, I’ll explain why RAID 5 is too risky for large drives, why RAID 6 is better but still not perfect, and what alternatives you should consider.


What is RAID 5?

RAID 5 is a single-parity RAID configuration, meaning it can tolerate one drive failure. When a drive fails, the RAID controller reconstructs its data using the parity information spread across the remaining drives.

It sounds great, right? But there’s a catch. Rebuilding a RAID 5 array requires reading all the remaining drives. If a single Unrecoverable Read Error (URE) occurs during the rebuild, the entire rebuild process fails, and you lose all your data. 😨


What are Unrecoverable Read Errors (UREs)?

A URE happens when a hard drive can’t read a sector of data. This isn’t a mechanical failure—it’s a bit-level issue where a drive simply fails to return the requested data.

Manufacturers publish URE rates as something like 1 in 10¹⁴ bits read for consumer drives, meaning there’s a 1 in 100 trillion chance of encountering an error. That sounds tiny… but modern drives are HUGE.

Why Are UREs a Problem for RAID?

  • A single 14TB drive contains 112 trillion bits.
  • If your RAID 5 array has 7 drives, you must read ~84TB of data during a rebuild.
  • With a 1 in 10¹⁴ URE rate, the chance of encountering a read error during rebuild is ~19%.

In other words, 1 in every 5 RAID 5 rebuilds will fail due to UREs. 😬


Why RAID 6 is (Mostly) Better

RAID 6 solves RAID 5’s weakness by adding a second parity disk, allowing it to tolerate two drive failures instead of just one.

How RAID 6 Handles UREs Better

  • If a URE occurs during a rebuild, RAID 6 can reconstruct the missing data using the second parity block.
  • You’re still at risk if multiple UREs hit the same stripe, but the odds of this happening are far lower than in RAID 5.

But RAID 6 Isn’t Perfect Either

  • Rebuild times are even longer than RAID 5 because of the extra parity calculations.
  • RAID 6 still isn’t immune to bit rot (silent corruption of stored data).
  • If you’re using very large drives (18TB+), RAID 6 might not be enough.

Verdict: RAID 6 is acceptable, but better alternatives exist if you want maximum data protection.


Better Alternatives: RAID-Z2 and Btrfs RAID

1. ZFS (RAID-Z2 or RAID-Z3) 🛡️

If you’re serious about data integrity, RAID-Z2 (ZFS with double parity) is a great alternative to RAID 6. It provides:

Self-healing—ZFS checksums every block and can automatically repair corrupt data.
Better URE handling—Instead of failing the rebuild, ZFS logs errors and continues.
No write hole problem—Traditional RAID 5/6 has a risk where power loss can corrupt parity data; ZFS avoids this.
RAID-Z3 available—For extreme redundancy, you can use triple parity (RAID-Z3).

🔹 Downside: Requires more RAM (ZFS loves memory).


2. Btrfs RAID 6 🛡️

Btrfs is another modern file system that improves on traditional RAID.

Handles UREs better—Like ZFS, Btrfs allows partial rebuilds instead of full failures.
Self-healing and checksumming—Prevents silent data corruption.
More flexible than ZFS—Supports online RAID resizing and bitwise scrubbing.

🔹 Downside: Btrfs RAID 5/6 is not as mature as ZFS RAID-Z2/3.


Final Recommendation: What Should You Use in Your Homelab?

Storage Setup Pros Cons
RAID 5 (Avoid!) Maximizes storage space High risk of URE-related failure
RAID 6 Safer than RAID 5, works well for medium setups Still vulnerable to long rebuild times
RAID-Z2 (ZFS) Best balance of protection & performance Requires more RAM
RAID-Z3 (ZFS) Extreme protection for mission-critical data Overkill for most homelabs
Btrfs RAID 6 Great alternative to RAID-Z2, flexible Not as mature as ZFS

My choice for a 7-bay UNAS-Pro in 2025? Well, the UNAS-Pro currently only offers RAID 1,5,6 and 10. For a large array I can only recommend using RAID 6. Everything else is to much of a risk!

If you ’re an advanced user with a custom setup, ZFS RAID-Z2 could be worth exploring If you’re dealing with super critical data, RAID-Z3 is an option, but for most homelabs, RAID-Z2 will do just fine.


Final Thoughts

RAID 5 was great… in the early 2000s. But with today’s massive HDDs, the risk of URE-induced failures makes RAID 5 a terrible choice. RAID 6 is better but still not bulletproof for long-term data protection.

If you’re setting up a 7-bay UNAS-Pro, you’re already serious about your data. Do yourself a favor—skip RAID 5 and go with RAID 6. Even better would be something designed for modern storage like RAID-Z2 or Btrfs RAID 6 but unfortunately this is not supported by Ubiquiti right now!


What’s Your Setup?

Are you still using RAID 5? Thinking of upgrading your storage? Let me know what you think or if you think I am completely wrong.