Size printed on the drive label. Different sizes? The array uses the smallest one for every drive.
Advanced settings
Typical hard drive: 100–200 MB/s with no other load. SSD: 400 MB/s or more.
From the drive datasheet. It is a worst-case spec, so real drives usually do better.
Drives that can fail
Rebuild of one drive
Read error during rebuild
Checks for this setup
The same drives in every RAID level
4 drives of 12 TB each.
How rebuild time is calculated
A rebuild writes one full drive. However large the array is, the new drive has to be filled from start to end, so the time depends on the size of one drive and the speed the system can sustain:
Typical hard drives sustain 150 to 250 MB/s at the outer edge of the platter and about half that at the inner edge, and the system keeps serving files during the rebuild. On a busy array, 50 MB/s is common. ZFS is the exception: it rebuilds only the data actually stored, so a half-full pool rebuilds in about half the time.
| Drive size | 50 MB/s | 100 MB/s | 150 MB/s | 200 MB/s | 250 MB/s |
|---|---|---|---|---|---|
| 2 TB | 11 h | 5 h 33 min | 3 h 42 min | 2 h 47 min | 2 h 13 min |
| 4 TB | 22 h | 11 h | 7 h 24 min | 5 h 33 min | 4 h 27 min |
| 6 TB | 33 h | 16 h | 11 h | 8 h 20 min | 6 h 40 min |
| 8 TB | 44 h | 22 h | 14 h | 11 h | 8 h 53 min |
| 10 TB | 2.3 days | 27 h | 18 h | 13 h | 11 h |
| 12 TB | 2.8 days | 33 h | 22 h | 16 h | 13 h |
| 14 TB | 3.2 days | 38 h | 25 h | 19 h | 15 h |
| 16 TB | 3.7 days | 44 h | 29 h | 22 h | 17 h |
| 18 TB | 4.2 days | 2.1 days | 33 h | 25 h | 20 h |
| 20 TB | 4.6 days | 2.3 days | 37 h | 27 h | 22 h |
| 22 TB | 5.1 days | 2.5 days | 40 h | 30 h | 24 h |
| 24 TB | 5.6 days | 2.8 days | 44 h | 33 h | 26 h |
The chance of a read error during a RAID 5 rebuild
RAID 5 and two-drive mirrors have nothing left to fix a sector that cannot be read during a rebuild. The table shows the chance of at least one such error during the rebuild of a four-drive RAID 5, at the worst-case error rates drive makers publish:
| Array | Read during rebuild | Desktop drives (10¹⁴) | NAS drives (10¹⁵) |
|---|---|---|---|
| 4 × 4 TB | 12 TB | 62% | 9.2% |
| 4 × 8 TB | 24 TB | 85% | 17% |
| 4 × 12 TB | 36 TB | 94% | 25% |
| 4 × 16 TB | 48 TB | 98% | 32% |
| 4 × 20 TB | 60 TB | 99% | 38% |
| 4 × 24 TB | 72 TB | over 99% | 44% |
RAID 6, RAID 60 and three-way mirrors still have one level of protection during a rebuild, so a read error is corrected.
Why large drives make rebuilds risky
Every hard drive has a tiny but non-zero chance of an unrecoverable read error (URE) — a sector it simply cannot read back. Manufacturers quote this as a rate, commonly around one error per 10^14 bits read for consumer drives. That sounds astronomically rare, and for everyday use it is. The problem appears during a parity rebuild.
To rebuild a failed drive in RAID 5, the controller must read every bit on all the remaining drives to recalculate the missing data. The larger the array, the more total data must be read flawlessly in one pass. As capacities climb into the tens of terabytes, the odds of hitting at least one URE somewhere during that read grow from negligible to meaningful. If it happens in RAID 5, the rebuild can fail.
This is the core argument behind dual-parity RAID 6 for large arrays: a single read error during rebuild is survivable because the second parity block fills the gap. It is also a strong reminder that healthy, regularly scrubbed drives and proper monitoring matter as much as the RAID level itself.
What happens during a rebuild
When a drive fails in a redundant array, the surviving drives keep serving data while a replacement is installed and reconstructed. The rebuild process sounds simple in theory: read every remaining block, recalculate the missing data, write it to the new drive. In practice, it is one of the most stressful periods your array will ever face, and understanding what happens during it is the difference between a smooth recovery and a catastrophic data loss.
The maths of rebuild time
A rebuild copies (or recalculates) the full capacity of one drive. If the controller sustains 100 MB/s during the operation, a 10 TB drive takes roughly 28 hours to complete. Double the capacity, double the time. A 20 TB drive at 100 MB/s means over two days of continuous sustained reads across every surviving disk in the array. During that window, none of the remaining drives get to rest, and any single unrecoverable read error on any of them can end the rebuild.
In real deployments, sustained rebuild speed is rarely at manufacturer peak. Array activity (users reading files, backups running, virtual machines writing to disk) competes with rebuild traffic for the same spindles. It is common to see rebuild speeds drop to 30–50 MB/s on production arrays, tripling the total time. A 20 TB rebuild on a busy array can genuinely take five or six days.
Why the array is at its most vulnerable during rebuild
Three things happen simultaneously during a rebuild that make failure more likely, not less:
Sustained heavy read load. Every surviving drive is read from end to end. If any of them was already on the edge of failure (marginal SMART attributes, borderline heat), the rebuild is often what pushes it over. This is why the second failure so often happens during the first rebuild.
Same age, same batch. Drives are frequently purchased at the same time from the same production batch, and they have run the same hours in the same environment. When one dies of old age, the others are statistically likely to be close behind. Rebuilding stresses them exactly when they are weakest.
URE probability accumulates over the read pass. The unrecoverable read error rate for consumer drives is typically quoted as 1 in 10^14 bits. That sounds tiny, but reading 10 TB of data means reading 8 × 10^13 bits — almost exactly the URE probability threshold. On a large RAID 5 array with several such drives, hitting one URE somewhere during the rebuild is not a rare event; it is a plausible outcome. RAID 6 survives it; RAID 5 does not.
Scenario: 4-drive RAID 5 array with 12 TB drives
One drive fails on a Tuesday morning. You replace it Tuesday afternoon; the rebuild starts. Estimated time at 60 MB/s sustained: about 55 hours — so the array runs in a degraded state through Wednesday, Thursday, and finishes Thursday evening.
During those 55 hours, the three surviving drives read a combined 36 TB of data. Any single URE on any of them fails the rebuild. If a second drive fails outright — also more likely because drives from the same batch tend to fail near each other — the array is lost.
This is why administrators building new large arrays rarely choose RAID 5. The rebuild window is simply too long and too risky. RAID 6 (which survives both a URE and a second complete drive failure during rebuild) or RAID 10 (which rebuilds by copying a single mirror rather than recalculating parity across the whole array, typically in a fraction of the time) are the practical choices.
How to reduce rebuild risk in practice
Rebuild time is not something you can eliminate, but you can shape the risk profile:
- Choose RAID 6 for drives 8 TB and above. The extra parity block is exactly what protects you during the long rebuild window.
- Run regular data scrubbing. Most controllers and software RAID implementations support periodic scrubs that read every block and quietly repair errors using parity before they become a problem during rebuild. Weekly or monthly scrubs on production arrays are standard practice.
- Monitor SMART aggressively. Warning signs (reallocated sectors, pending sectors, elevated read error rates) usually appear before hard failure. Replace drives showing warnings before they fail, so the rebuild happens on your schedule with fresh surrounding drives.
- Keep hot spares. A hot spare drive already installed in the chassis begins rebuilding automatically the moment a drive drops out. Even a few hours of shorter degraded time meaningfully reduces the exposure window.
- Do not mix batches. When building a new array, buying drives from multiple production batches (or even different vendors of the same specification) reduces the correlated-failure risk.
Questions
How long does it take to rebuild a 16 TB drive?
About 30 hours at 150 MB/s with no other load. On a busy array at 50 MB/s, about 3.7 days.
Does a bigger array take longer to rebuild?
Not much. The rebuild always writes one drive, so its size sets the time. A larger parity array reads more data during the rebuild, which raises the risk, not the time.
Can I use the array during a rebuild?
Yes, the array stays online, but it is slower and the rebuild takes longer. If you can, keep heavy work for after the rebuild.
How do I make rebuilds safer?
Use RAID 6 or RAID 10 with large drives, scrub the array regularly, watch SMART warnings, keep a hot spare and keep a separate backup.