RAID 5 or RAID 6 With Large Hard Drives: Which Should I Use?
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RAID 5 survives one drive failure and RAID 6 survives two, in the words of Synology, QNAP and Asustor. Synology recommends RAID 6 when an array has more than 7 drives, and for large arrays where a second failure is more likely. Drive data sheets state a non-recoverable read error rate per bits read, set by model number, and no maker here turns that into a chance that a rebuild fails.
Check your situation
Safe steps
These actions can lose data. Do none of them until a copy exists elsewhere:
- A second drive failing while a RAID 5 array is missing one. UGREEN says the array has no fault tolerance in that state, and if another drive fails, all data may be lost.
- The rebuild itself. UGREEN says a rebuild reads every remaining drive in full, which places significant stress on them, and that reading a latent bad sector then may trigger an Unrecoverable Read Error (URE) that can make the rebuild fail and may cause complete storage pool failure.
- Keep a current backup of anything you cannot lose, whichever level you pick.
- Count your drives, then check how many failures your maker says the level survives.[1]
- Read the non-recoverable read error rate on the data sheet for each exact model number, not for the product line.[8]
- Estimate how long a rebuild would take with the rebuild time calculator and the rate your NAS reports.
What to buy, if anything
Answer the questions under "Check your situation" and this section narrows to what your case calls for. Until then every table is shown, each with the case it is for.
Large NAS and enterprise drives rated at most 1 error per 10^15 bits read
Why this is here: For a reader building or expanding an array of large drives who wants each drive's maker-stated non-recoverable read error rate on record; these drives' data sheets state the 10^15 figure. The table does not make RAID 5 safe or unsafe: how many drive failures an array survives is set by the RAID level (facts on Synology, QNAP, Asustor, UGREEN). New 3.5-inch SATA NAS and enterprise hard drives whose maker confirms CMR, listed only when the listing names a model number whose data sheet states a non-recoverable read error rate of 1 per 10^15 bits read (or less than 1 in 10^15), with today's Amazon.com price per TB.
NAS drives with a stated non-recoverable read error rate
Why this is here: For a reader who wants any drive whose maker states its non-recoverable read error rate, including the 10^14-rated NAS lines; the rate differs by model number, so the reader checks the data sheet for the exact model. New 3.5-inch SATA NAS and enterprise hard drives whose maker confirms CMR, listed only when the listing names a model number whose data sheet states a non-recoverable read error rate (10^14 or 10^15 bits), with today's Amazon.com price per TB.
Frequently asked questions
What is the chance that a RAID 5 rebuild fails on large drives?
No maker in this page's sources states one, and this page does not compute one. Data sheets give only a non-recoverable read error rate per bits read, in each maker's own terms.
Does SHR-2 protect like RAID 6?
Synology says SHR-2 provides 2-drive fault tolerance when enough drives are installed, and SHR-1 provides 1-drive fault tolerance.
Sources
- Synology knowledge center: What RAID type is best for my storage? checked 2026-10-14
- Synology DSM help: What is RAID? checked 2026-10-14
- Synology DSM help: Repair a Storage Pool checked 2026-10-14
- QNAP QTS 5.2.x user guide: RAID types checked 2026-10-14
- Asustor College NAS 251: Introduction to RAID checked 2026-10-14
- UGREEN knowledge center: Storage Pool Expansion Guide (Replacing Drives One by One) checked 2026-10-14
- UGREEN knowledge center: [FAQ] What to Do if the RAID Sync Speed is Slow in the Storage? checked 2026-10-14
- Seagate IronWolf data sheet DS1904.23-2509 checked 2026-10-14
- Seagate IronWolf Pro data sheet DS2129.4-2311 checked 2026-10-14
- Seagate Exos X24 data sheet DS2080-2307 checked 2026-10-14
- Western Digital: WD Red Plus NAS hard drive product brief checked 2026-10-12
- Western Digital: WD Red Pro NAS hard drive data sheet checked 2026-10-12
- Western Digital: Ultrastar DC HC550 data sheet checked 2026-10-12
Facts on this page were reviewed on October 14, 2026. Prices and stock are from Amazon.com as of October 14, 2026.