Too slow · Reviewed October 14, 2026 · Prices updated 1 hour ago

Why Is My NAS Rebuild Taking So Long, and Is SMR the Cause?

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Back up before you repair. If the data on this system exists nowhere else, copy what matters to another device first, while the drives still read.

Quick answer

No maker in this guide publishes an expected rebuild speed except UGREEN, which says syncing 1TB typically takes about 4 hours on its systems. Check the rebuild priority setting, how full the pool is and what else the NAS is doing; changing those needs nothing bought. Only TrueNAS ties SMR drives to longer resilvers, so check each drive's model number before you buy anything.

Reviewed October 14, 2026

Check your situation

Does the NAS show the rebuild running and making progress?

On QNAP, a running rebuild shows the status Rebuilding with its progress.

No maker here gives a rebuild speed, except UGREEN. Synology gives no speed figure. It says actual speed may vary depending on models and drives. QNAP says more disks in a RAID group lengthen rebuild times. TrueNAS says a resilver of a pool with a large amount of data can take a long time. Neither gives a number. Asustor says the time depends on drive capacity and the amount of data stored, and that syncing a single drive may take a couple of hours. UGREEN gives figures for its own systems: syncing 1TB typically takes about 4 hours, rebuilding more than 10 TB typically takes 10 to 20 hours or longer, and replacing all four drives of a four-drive array one by one may take 3 to 4 days. These are UGREEN's figures, not a general rate. To estimate your own, read the rebuild rate your NAS shows and enter it in the rebuild time calculator with the drive size and how full the array is. The site claims no rebuild rate for any NAS or drive. Use the whole-drive figure unless your maker documents that the rebuild skips unused space. Synology's Fast Repair, below 80% pool usage, and UGREEN's Quick Repair are documented that way.

Get the rebuild started. QNAP says that in some cases the rebuild does not start on its own. Set the new drive as a spare disk and QNAP says the rebuild starts at once. On TrueNAS, replacing a heavily degraded disk without taking it offline first significantly slows the replacement. Take that disk offline before you replace it. Nothing needs buying for either step.

Raise the rebuild priority. Synology: go to Storage > Global Settings > RAID Resync Speed Limits. This one setting covers repairs, drive replacement, scrubbing and expansion. The option Synology marks as recommended lowers the impact on system performance. Choose Run RAID resync faster, or Customize, where Max is the speed when the system is idle and Min is the maximum speed while there is I/O. QNAP: set RAID Resync Priority in Storage global settings. Resync First runs RAID operations at higher speeds, and storage may feel slower while they run. Service First runs them at lower speeds. QNAP says the setting only matters while the NAS is in use; when it is idle, RAID operations run at the highest possible speeds. UGREEN: in UGOS Pro, go to Storage > Storage > Advanced Settings and, under Performance, select Prioritize RAID sync. When the sync finishes, switch back to Prioritize user read/write. TrueNAS 25.10: in Advanced Settings, select Run Resilvering At Higher Priority At Certain Times and set the days and time range. TrueNAS's CORE 13.0 documentation says a resilver in that window likely runs faster than when demand on the system is higher. Clear the checkbox and save to return to the default. Unraid: starting the array in Maintenance Mode can speed up a rebuild, which you then start with Sync on the Main tab. The array is not accessible while it runs. None of this needs anything bought.

Lighten the load on the pool. Synology's Fast Repair skips unused space and is on by default. Above 80% pool usage, DSM switches to Regular Repair on its own, so the whole-drive figure in the rebuild time calculator is the one that applies. UGREEN's Enable Quick Repair option also skips unused space in a degraded pool. Cut other work until the rebuild ends. UGREEN recommends no file writes during a RAID sync and as few large reads and writes as possible during a rebuild. TrueNAS's CORE 13.0 documentation advises avoiding replications, SMB, NFS and Rsync transfers, SMART tests and pool scrubs during a resilver. Nothing needs buying.

Plan to replace SMR drives with CMR, one at a time, after this rebuild. TrueNAS's SCALE Hardware Guide says drive-managed and host-aware SMR drives function poorly with TrueNAS and are not recommended. It says to expect degraded write performance and longer, less reliable resilvers. Host-managed SMR drives are not supported. TrueNAS is the only source here that ties SMR to slower resilvers. Synology says something different: do not build an array from a mixture of PMR (CMR) and SMR drives. Let the current rebuild finish first. Then replace each SMR drive with a CMR drive, one drive at a time, and wait for each rebuild to end before the next. The table below lists only drives whose maker confirms CMR.

Check for mixed drive types, then let it run. QNAP says a RAID group that combines different disk types (HDD, SSD, SAS) works at the speed of its slowest disk. UGREEN says mixing 4Kn and 512e sector formats in one RAID group may significantly reduce performance. If the SMR tool says the maker does not state the recording type, TrueNAS says to ask the drive maker, such as Western Digital or Seagate. If none of this applies, nothing needs buying. Let the rebuild finish, and estimate the time with the rebuild time calculator using the rate your NAS shows.

Work it out for your hardware

Try: , , , ,

The rules this tool applies, with their sources, are on its own page: Is My Drive SMR?.

Safe steps

These actions can lose data. Do none of them until a copy exists elsewhere:

Removing a disk or powering off the NAS while a rebuild or sync runs. QNAP warns against both.
Replacing a second drive before the first rebuild finishes. Asustor says replacing several drives at once, or any drive while the NAS is synchronizing, causes data loss. Unraid says that with single parity only one disk can be replaced at a time.
Another drive failing while the array is missing one. UGREEN says a RAID 5 pool has no fault tolerance in that state and all data may be lost. Unraid says the same risk applies to single parity during a rebuild.
UGREEN says a latent bad sector read during the rebuild may trigger an Unrecoverable Read Error (URE) that can make the rebuild fail and may cause complete storage pool failure.
A power outage during the rebuild. UGREEN says it may corrupt logical sectors and stop the file system mounting, and recommends a UPS.

Back up anything you cannot afford to lose before you change any setting.
Leave every disk in place and keep the NAS powered on until the rebuild finishes.^[7]
Check the rebuild priority setting first. Changing it costs nothing.^[2]
Hold off on file writes and large transfers until the rebuild ends.^[8]
Keep the NAS on a UPS during the rebuild.^[9]
Check every drive's model number with the Is My Drive SMR? tool before you buy a replacement.

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.

CMR NAS drives for replacing an SMR drive

Why this is here: Only for the branch where the maker's own document shows a drive in the array is SMR: TrueNAS does not recommend drive-managed SMR drives and expects longer resilvers with them, and Synology says not to build an array from a mix of PMR (CMR) and SMR drives. A rebuild that is slow only because of its priority setting, pool usage or other load needs nothing bought. New 3.5-inch SATA NAS and enterprise hard drives whose maker confirms CMR recording, with today's Amazon.com price per TB.

Price per

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30 products

Product	Capacity	Price	$ / TB	Type	Prime	Buy
DC HC550 16TB 512MB SATA Ultra SE NP3	16 TB	$589.45	$36.84	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate IronWolf 14TB NAS Internal Hard Drive HDD – CMR 3.5 Inch SATA 256MB Cache for RAID Network Attached Storage – Frustration Free Packaging (ST14000VN0008)	14 TB	$525.00	$37.50	NAS HDD · SATACMR	-	Buy on Amazon
TOSHIBA N300 Internal Hard Drive 22 Tb 7200 RPM 512 Mb 3.5", W129161818 (Tb 7200 RPM 512 Mb 3.5 Serial Ata Iii)	22 TB	$830.82	$37.76	NAS HDD · SATACMR	-	Buy on Amazon
Toshiba - HDWG82AXZSTB - Toshiba N300 Pro HDWG82AXZSTB 20 TB Hard Drive - 3.5 Internal - SATA (SATA/600) - Conventional	20 TB	$766.99	$38.35	NAS HDD · SATACMR	-	Buy on Amazon
Toshiba MG Series Enterprise 12TB 3.5’’ SATA 6Gbit/s Internal HDD 7200RPM 550TB/year 24/7 Operation. MG07ACA12TE	12 TB	$469.99	$39.17	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate IronWolf Pro, 16 TB, Enterprise NAS Internal HDD –CMR 3.5 Inch, SATA 6 Gb/s, 7,200 RPM, 256 MB Cache for RAID Network Attached Storage (ST16000NT001)	16 TB	$629.99	$39.37	NAS HDD · SATACMR	-	Buy on Amazon
Seagate Enterprise 3.5" 18000 GB 7200 RPM 18 TB 3.5" 7200 RPM SATA ST18000NM00J	18 TB	$712.00	$39.56	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate Enterprise Capacity \| ST10000NM0016 \| 10TB 7200 RPM SATA 6.0Gb/s 256MB Cache Hyperscale 512e 3.5" Hard Disk Drives	10 TB	$399.95	$39.99	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Western Digital WUH721414ALE6L4 14TB Ultrastar DC HC530 SATA HDD - 7200 RPM Class, SATA 6 Gb/s, 512MB Cache, 3.5"	14 TB	$581.00	$41.50	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate 20TB Exos Enterprise Hard Drive \| SATA (ST20000NM002H) \| Enterprise nearline HDD for AI, cloud & data centers. Always-on performance, helium sealed, 2.5M-hr MTBF. 5-Year Warranty	20 TB	$829.99	$41.50	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Toshiba - HDWG81GXZSTB - Toshiba N300 Pro HDWG81GXZSTB 16 TB Hard Drive - 3.5 Internal - SATA (SATA/600) - Conventional	16 TB	$685.50	$42.84	NAS HDD · SATACMR	-	Buy on Amazon
Seagate 14TB HDD Exos X14 7200 RPM 512e/4Kn SATA 6Gb/s 256MB Cache 3.5-Inch Enterprise Hard Drive (ST14000NM0018)	14 TB	$600.00	$42.86	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
HGST WD Ultrastar DC HC510 10TB 7200 RPM SATA 6Gb/s 3.5" Helium Platform Enterprise Hard Disk Drive - HUH721010ALE604 (0F27606)	10 TB	$429.00	$42.90	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate Exos X18 Enterprise Class, 16TB, Enterprise Internal Hard Drive, SATA, 3,5" 6Gbit/s, 128MB Cache (ST16000NM000J)	16 TB	$686.91	$42.93	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate IronWolf 10TB NAS Internal Hard Drive, 3.5" SATA 6Gb/s, 7200 RPM, CMR, RAID Optimized, IronWolf Health Management – for NAS Systems (ST10000VNZ00)	10 TB	$432.99	$43.30	NAS HDD · SATACMR	-	Buy on Amazon
Western Digital 22TB WD Red Pro NAS Internal Hard Drive HDD - 7200 RPM, SATA 6 Gb/s, CMR, 512 MB Cache, 3.5" - WD221KFGXSold by Amazon	22 TB	$958.90	$43.59	NAS HDD · SATACMR	Prime	Buy on Amazon
Seagate Exos X10 ST10000NM0126 10TB 7200 RPM SATA 6Gb/s 512e 256MB 3.5in Enterprise Hard Drive	10 TB	$439.99	$44.00	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Western Digital 20TB WD Red Pro NAS Internal Hard Drive HDD - 7200 RPM, SATA 6 Gb/s, CMR, 512 MB Cache, 3.5" - WD202KFGXSold by Amazon	20 TB	$881.68	$44.08	NAS HDD · SATACMR	Prime	Buy on Amazon
Western Digital 18TB WD Red Pro NAS Internal Hard Drive HDD - 7200 RPM, SATA 6 Gb/s, CMR, 512 MB Cache, 3.5" - WD181KFGX	18 TB	$794.99	$44.17	NAS HDD · SATACMR	-	Buy on Amazon
Western Digital 24TB WD Red Pro NAS Internal Hard Drive HDD, 7200 RPM, SATA 6 Gb/s, CMR, 512 MB Cache, 3.5" - WD241KFGXSold by Amazon	24 TB	$1,059.99	$44.17	NAS HDD · SATACMR	Prime	Buy on Amazon
Seagate IronWolf Pro 14TB NAS Internal Hard Drive HDD – CMR 3.5 Inch SATA 6Gb/s 256MB Cache for RAID Network Attached Storage, Data Recovery Service – Frustration Free Packaging (ST14000NEZ008)	14 TB	$619.99	$44.29	NAS HDD · SATACMR	-	Buy on Amazon
Seagate IronWolf 16TB NAS Internal Hard Drive HDD – CMR 3.5 Inch SATA 6GB/S 7200 RPM 256MB Cache for Raid Network Attached Storage, with Rescue Service (ST16000VN001)	16 TB	$711.00	$44.44	NAS HDD · SATACMR	-	Buy on Amazon
Seagate IronWolf Pro ST12000NE0008 12 TB Hard Drive - 512E Format - SATA 600-3.5" Drive - Internal - 7200RPM - 256 MB Buffer - Hot Pluggable	12 TB	$534.99	$44.58	NAS HDD · SATACMR	-	Buy on Amazon
Seagate Exos X14 ST12000NM0008 12 TB Hard Drive - 3.5" Internal - SATA (SATA/600)	12 TB	$535.68	$44.64	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Western Digital 16TB WD Red Pro NAS Internal Hard Drive HDD - 7200 RPM, SATA 6 Gb/s, CMR, 512 MB Cache, 3.5" - WD161KFGXSold by Amazon	16 TB	$714.99	$44.69	NAS HDD · SATACMR	Prime	Buy on Amazon
Western Digital 12TB WD Red Plus NAS Internal Hard Drive HDD - 7200 RPM, SATA 6 GB/s, CMR, 512 MB Cache, 3.5" - WD120EFGX	12 TB	$538.88	$44.91	NAS HDD · SATACMR	-	Buy on Amazon
Western Digital 12TB WD Red Pro NAS Internal Hard Drive HDD - 7200 RPM, SATA 6 Gb/s, CMR, 512 MB Cache, 3.5" - WD122KFBXSold by Amazon	12 TB	$539.16	$44.93	NAS HDD · SATACMR	Prime	Buy on Amazon
Seagate IronWolf 10Tb NAS Internal Hard Drive HDD – 3.5 Inch SATA 6GB/S 7200 RPM 256MB Cache for Raid Network Attached Storage (ST10000VN0004)	10 TB	$449.99	$45.00	NAS HDD · SATACMR	-	Buy on Amazon
HGST - WD Ultrastar DC HC520 HDD \| HUH721212ALE600 \| 12TB 7.2K SATA 6Gb/s 256MB Cache 3.5-Inch Helium Data Center Internal Hard Disk Drive	12 TB	$541.45	$45.12	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Western Digital 10TB WD Red Plus NAS Internal Hard Drive HDD - 7200 RPM, SATA 6 GB/s, CMR, 512 MB Cache, 3.5" - WD100EFGXSold by Amazon	10 TB	$455.75	$45.58	NAS HDD · SATACMR	Prime	Buy on Amazon

Frequently asked questions

Does Synology say SMR drives slow a rebuild?

No. Synology's page says only not to build an array from a mixture of PMR (CMR) and SMR drives. Among the sources here, only TrueNAS ties SMR drives to longer resilvers.

Sources

Synology DSM help: Repair a Storage Pool checked 2026-10-14
Synology DSM help: Adjust the RAID Resync Speed Limits checked 2026-10-14
Synology knowledge center: What can I do with low data transfer rate? checked 2026-10-14
QNAP QTS 5.2.x user guide: Storage global settings checked 2026-10-14
QNAP QTS 5.2.x user guide: RAID types checked 2026-10-14
QNAP FAQ: My RAID (RAID1, RAID5, RAID6) is in degraded mode. What should I do? checked 2026-10-14
QNAP QTS 5.2.x user guide: Expanding a RAID group by replacing all disks checked 2026-10-13
UGREEN knowledge center: [FAQ] What to Do if the RAID Sync Speed is Slow in the Storage? checked 2026-10-14
UGREEN knowledge center: Storage Pool Expansion Guide (Replacing Drives One by One) checked 2026-10-14
UGREEN knowledge center: Storage checked 2026-10-14
Asustor College NAS 352: RAID Level Migration and Capacity Expansion checked 2026-10-14
TrueNAS 25.10 documentation: Replacing Disks checked 2026-10-14
TrueNAS 25.10 documentation: Advanced Settings (Changing Resilvering Priority) checked 2026-10-14
TrueNAS CORE 13.0 documentation: Using Resilver Priority checked 2026-10-14
TrueNAS 25.10 documentation: TrueNAS Hardware Guide checked 2026-10-14
Unraid documentation: Replacing disks in array 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.