A red exclamation mark showed up in my Synology dashboard one Tuesday morning, and the little tooltip said something about reallocated sectors. My stomach dropped. I assumed the drive was seconds from death and I was about to lose four years of family photos.
It was not. That drive kept running for another two and a half years before I retired it on my own schedule.
Bad sectors are one of the most misunderstood alerts a home NAS owner ever sees. Some of them are genuinely a warning to move fast, and some of them are background noise your drive has already handled. The whole trick is knowing which is which, and that comes down to reading a few specific numbers correctly.
What actually is a bad sector?
A sector is the smallest chunk of storage on a hard drive, historically 512 bytes and now usually 4 kilobytes on modern disks. A bad sector is simply one that the drive can no longer reliably read from or write to.
They come in two flavors, and the difference matters more than almost anything else in this article. A physical bad sector is real damage to the platter surface, often from a head touching the disk or a manufacturing flaw. A logical or soft bad sector is a spot where the data got scrambled, usually from a power loss during a write, and it can often be fixed by writing fresh data over it.
Your drive deals with physical bad sectors quietly. Every hard drive ships with a pool of spare sectors set aside from the factory. When it finds a bad one, it copies the data elsewhere and swaps in a spare. That swap is called a reallocation.
My drive shows reallocated sectors. Is it failing?
Not necessarily, and this is the number people panic over the most. The SMART attribute you want is Reallocated Sectors Count, ID 5 in the raw data.
A handful of reallocated sectors on a drive that has run for years is normal wear. I have seen healthy 8TB drives sit happily at a raw value of 8 or 16 for their entire life without ever getting worse. The disk found a few weak spots early, swapped them out, and moved on.
The signal to worry about is not the count itself. It is the trend.
If you went from 0 to 40 reallocated sectors in a month, that drive is shedding surface and you should plan a replacement. If it went from 8 to 8 over a year, relax and keep an eye on it.
What about pending or uncorrectable sectors?
This is where I get much more nervous, and you should too. Two attributes matter here.
Current Pending Sector Count (ID 197) is a sector the drive tried to read, failed, and has flagged but not yet reallocated. It is in limbo. The drive is waiting for the next write to that spot to decide whether it can be reused or needs a spare.
Offline Uncorrectable (ID 198) is a sector the drive could not read and could not fix. That often means real data loss for whatever file lived there.
Here is the good news about pending sectors on a NAS specifically. Running a full disk scan, or in some cases a RAID scrub, forces the array to rewrite those spots. A pending sector often clears itself: it either gets rewritten cleanly and drops back to zero, or it converts to a reallocated sector. Either outcome is progress.
How do I actually read these numbers?
On a Synology box, open Storage Manager, click your drive under HDD/SSD, and open the Health Info tab, then the S.M.A.R.T. section. QNAP puts it under Storage and Snapshots, then Disks/VJBOD. TrueNAS users can run the check from the shell.
If you want the raw data on any Linux-based NAS, SSH in and run smartctl -a /dev/sda (swap the device letter). The three lines to find are below.
| SMART attribute | ID | What a rising value means |
|---|---|---|
| Reallocated Sectors Count | 5 | Watch the trend, a stable low number is fine |
| Current Pending Sector Count | 197 | Active read failures, back up now and scan |
| Offline Uncorrectable | 198 | Real data loss risk, plan replacement |
The manufacturer tools give you the same data with friendlier labels. Backblaze publishes years of real-world drive failure statistics built on exactly these attributes, and their breakdown of which SMART stats predict failure is the best plain-English reference I know of.
When should I definitely replace the drive?
I use a short mental checklist, and I have replaced drives I probably could have squeezed more life out of because the cost of being wrong is your data. Any one of these gets the disk retired.
- Pending or uncorrectable sectors that keep climbing after a full scan.
- Reallocated count that grows by more than a few sectors per week.
- The NAS logs read or write errors for that drive, not just SMART warnings.
- You hear clicking, grinding, or repeated spin-up sounds.
- The drive drops out of the array and needs a manual re-add.
A single replacement drive costs somewhere between 90 and 200 dollars for a typical 4TB to 8TB NAS-rated model as of mid 2026. Your photos, tax records, and backups are worth far more than that. When the numbers are trending the wrong way, this is not the place to save money.
Does running scans make the problem worse?
This is a fair worry, and the honest answer is mostly no. A read-heavy scrub or extended SMART test does put the drive under load, and a drive that is truly on the edge can fail during that load. But it was going to fail anyway, and better it happens while your backup is fresh and you are watching.
I run an extended SMART self-test monthly and a RAID scrub quarterly on my own gear. The scrub is the part that rewrites pending sectors and keeps the array honest. If you have never scheduled one, that is worth fixing today.
What you should not do is ignore a growing pending count and hope it heals on its own. Hoping is not a maintenance strategy.
The short version to carry with you
Reallocated sectors that sit still are the drive doing its job. Pending and uncorrectable sectors that climb are the drive asking for help. The single most useful habit is checking the trend over time rather than reacting to one scary number.
Keep a current backup, watch the counts monthly, and replace on the trend rather than on the fear. Do that and a bad sector alert becomes a heads-up instead of an emergency.
Frequently asked questions
How many bad sectors are acceptable on a NAS drive?
There is no single magic number, but a handful of stable reallocated sectors (single digits to low double digits) on a drive that has run for years is usually fine. What matters is whether the count is holding steady or growing. A count that climbs week over week means the drive is degrading and should be replaced.
Can bad sectors on a NAS drive spread to other drives?
No. Bad sectors are physical or logical damage on one specific drive and cannot jump to another disk. However, if one drive in a RAID array fails, the extra load during a rebuild can stress the remaining drives, which is why replacing a failing disk promptly matters.
Will a RAID scrub fix bad sectors?
A scrub can fix pending and soft bad sectors by rewriting the data to those spots, which either clears them or forces a clean reallocation. It cannot repair genuine physical surface damage, but it does resolve the sectors that were only in limbo. Running one quarterly is good practice.
Should I replace a drive as soon as SMART shows a warning?
Not automatically. Check which attribute triggered the warning first. Stable reallocated sectors rarely need urgent action, but rising pending or uncorrectable counts do. Confirm your backup is current, run a full scan, and replace the drive if the counts keep growing.
Does my NAS keep working with bad sectors?
Usually yes, at least for a while. The drive reallocates bad spots to spare sectors automatically, and in a RAID array with redundancy your data stays protected even if one drive struggles. The risk is that you are now running closer to the edge, so treat it as borrowed time and plan a replacement.
