How Do I Upgrade My Unraid Parity Drive to a Larger Disk?
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Only a disk larger than your current parity disk calls for a parity change. Unraid's documented order is to put the larger disk in the parity slot first and let parity build on it; the old parity disk can then become a data disk, and retiring it instead is also possible. If a data drive has already failed, Unraid says to use its Parity Swap procedure instead of this upgrade.
Check your situation
Work it out for your hardware
Synology DS923+: 4 bays, drives up to 24TB each, from Synology DS923+ datasheet. Drive compatibility for this model.
Path 1: swap drives one at a time
Space first rises at swap 2. The swaps before it gain nothing you can use. Each swap is one rebuild of a 8TB array member: 11.1 hours at 200MB a second, 22.2 hours at 100MB a second.
| Drives swapped | Usable space | Gained | Rebuild time so far |
|---|---|---|---|
| 1 of 4 | 24TB | nothing yet | 11.1 hours to 22.2 hours |
| 2 of 4 | 32TB | +8TB | 22.2 hours to 44.4 hours |
| 3 of 4 | 40TB | +16TB | 33.3 hours to 2.8 days |
| 4 of 4 | 48TB | +24TB | 44.4 hours to 3.7 days |
The two rates are yours: read them from your NAS during a rebuild or a scrub. A system that rebuilds only the used space finishes sooner. The last step, expanding onto the new space, takes extra time this tool cannot estimate.
- Synology supports replacing drives to expand SHR, SHR-2, RAID 1, RAID 5, RAID 6, RAID 10 and RAID F1 pools.
- Replace the smallest drive first. When every drive is the same size, at least two have to be replaced with larger ones before the pool can grow.
Path 2: add drives to empty bays
All 4 bays are in use, so there is nowhere to add a drive.
Path 3: back up, start again with 4 x 16TB
A new array of 4 x 16TB in SHR (Synology Hybrid RAID) holds 48TB, the same as the end of path 1, reached in one build instead of 4 rebuilds. It needs a complete copy of your data somewhere else first, because creating the new array erases the drives. At today's cheapest 16TB drive, 4 drives cost $2,357.80.
16TB drives that fit, by price
New 3.5-inch SATA drives rated for NAS or enterprise use whose maker confirms CMR recording, no larger than this model's 24TB per-bay limit. Prices are live from Amazon.com. As an Amazon Associate the site earns from qualifying purchases.
The rules this tool applies, with their sources, are on its own page: NAS Upgrade Advisor.
Safe steps
These actions can lose data. Do none of them until a copy exists elsewhere:
- With one parity disk, your data is unprotected until the parity build on the new disk is complete. If a data drive fails in that time, what is on it could be lost.
- Running the parity upgrade when a data drive has already failed will stop you from rebuilding that drive.
- Taking the old parity disk out before the new parity build completes removes the disk that can help recover data if a data disk fails during the upgrade.
- Formatting erases everything on a disk and cannot be undone. Never format a drive during a Parity Swap or when Unraid prompts during a rebuild.
- Starting a Parity Swap while another disk is failing or unhealthy increases the risk of data loss.
- Rebuilding a data disk when parity is not valid will corrupt that disk's file system.
- Pick a new parity disk at least as large as your largest data disk. Unraid's example: if the largest data disk is 10TB, parity should be at least 10TB.[1]
- Stop the array.[1]
- Power down the server. Skip this if your server supports hot-swap.[1]
- Install the new, larger disk, then power up.[1]
- Assign the new disk to the parity slot, replacing the old one. A parity disk needs no clearing.[1]
- Start the array. Unraid builds parity on the new disk. Your data stays accessible, but it may be slower until the build completes.[1]
- Keep the old parity disk installed until the new parity build completes. If a data disk fails during the upgrade, the old parity disk can help you recover the data.[1]
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.
Larger hard drives for a new parity disk
Why this is here: Unraid requires a parity disk at least as large as the largest data disk, so a disk larger than the current parity disk goes into the parity slot first; these are candidates for that new parity disk. New 3.5-inch SATA NAS and enterprise hard drives of 12TB and up whose maker confirms CMR, sorted by price per TB.
Frequently asked questions
I have two parity disks. What changes?
Both parity disks must be at least as large as your largest data disk, and they can be different sizes from each other. Unraid's parity upgrade section does not say whether to replace the two together or one after the other.
How long does the new parity build take?
Unraid gives no fixed figure. A parity sync reads every data disk and creates parity from scratch, and Unraid says parity work can take several hours depending on disk size and how busy the system is. The NAS Upgrade Advisor gives a time range from a rate you set; it claims no rate for any drive or server.
Sources
- Unraid documentation: Adding disks to array (includes Upgrading parity disks) checked 2026-10-14
- Unraid documentation: Replacing disks in array (includes Parity swap) checked 2026-10-14
- Unraid documentation: Array health and maintenance (parity checks and parity sync) checked 2026-10-14
Facts on this page were reviewed on October 14, 2026. Prices and stock are from Amazon.com as of October 14, 2026.