Plan a NAS Storage Upgrade
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There are three ways to grow a full NAS: swap the drives for larger ones one at a time, add drives to empty bays, or back everything up and start again with a new set. Which one works depends on the array type, and on most RAID types swapping one drive gains nothing until the last drive is in. Enter what you have and the tool shows the usable space each path gives, the number of rebuilds it takes, and the drives that fit with today's price per TB.
These actions can lose data. Do none of them until a copy exists elsewhere:
- Pulling a second drive before the first rebuild has finished.
- Replacing a drive in a RAID 5, SHR or RAID-Z1 array while another drive in it is already failing: the array has no protection left during the rebuild.
- Wiping the old drives during a full swap before the restored copy has been checked.
Your NAS and what you are thinking of buying
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.[1]
- 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.[1]
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.
Worked example: four 8TB drives swapped for 16TB
The same four swaps give space back at different points depending on the array type. SHR grows from the second swap. RAID 5 and RAID 6 give nothing until the fourth.
| Drives swapped | SHR | RAID 5 | RAID 6 |
|---|---|---|---|
| None (today) | 24TB | 24TB | 16TB |
| 1 of 4 | 24TB | 24TB | 16TB |
| 2 of 4 | 32TB | 24TB | 16TB |
| 3 of 4 | 40TB | 24TB | 16TB |
| 4 of 4 | 48TB | 48TB | 32TB |
How the figures are worked out
Usable space is drive capacity before filesystem overhead, in the TB printed on the drive. RAID 1 stores one drive's worth. RAID 5 stores the number of drives minus one, times the smallest drive. RAID 6 stores the number of drives minus two, times the smallest drive. RAID 10 stores half the drives, times the smallest drive. Synology and Asustor both publish these formulas.
SHR stores the total minus the largest drive, and SHR-2 the total minus the two largest, which is the arithmetic the site's Synology RAID calculator uses. RAID-Z1, Z2 and Z3 are worked like RAID 5 and 6 with one, two or three drives of parity. On Unraid every data disk counts in full, the parity disks hold no data, and no data disk may be larger than a parity disk.
The time range is the rebuild calculator's method: the size of the drive being rebuilt, in megabytes, divided by the rate, divided by 3,600. The two rates are yours to set. Read them from your NAS during a rebuild or a scrub. No rate is claimed here for any NAS or drive.
What the tool does not know
- How long the final expansion step takes once the last drive is in. Makers do not publish a rate for it.
- How long adding a drive takes. That is a reshape of the whole array, not a rebuild of one drive.
- TerraMaster TRAID capacity. TerraMaster publishes no formula for it.
- Arrays that already mix drive sizes. The tool assumes your current drives are all the same size.
Sources
- Synology DSM help: Replace a Drive (expand a storage pool by replacing drives) checked 2026-10-13
- Synology DSM help: Add Drives to Expand the Storage Pool Capacity checked 2026-10-13
- QNAP QTS 5.2 user guide: Expanding a RAID group by replacing all disks checked 2026-10-13
- QNAP QTS 5.2 user guide: Expanding a static volume by adding disks to a RAID group checked 2026-10-13
- UGREEN knowledge center: Storage Pool Expansion Guide (Replacing Drives One by One) checked 2026-10-13
- UGREEN knowledge center: Storage checked 2026-10-13
- Asustor College NAS 352: RAID Level Migration and Capacity Expansion checked 2026-10-13
- TrueNAS 25.10 documentation: Managing Pools checked 2026-10-13
- Unraid documentation: Adding disks to array checked 2026-10-13
- Unraid documentation: Replacing disks in array checked 2026-10-14
The rules on this page were reviewed on October 13, 2026. Prices and stock are from Amazon.com as of October 13, 2026.