Guide
How to tell your hard drive is starting to fail.
Drives don't usually fail without warning. Most give measurable signals before they stop working entirely — the problem is that few people know where to look before it's too late.
Signs you can notice without technical tools
Before checking technical data, there are warning signs you can notice without specialized software: new, repetitive noises on mechanical drives (clicking, grinding, buzzing), files that get corrupted or won't open for no apparent reason, the system freezing or taking a long time to access certain files, the computer intermittently failing to recognize the drive, or a noticeable drop in transfer speed compared to how the drive used to behave. No single sign is conclusive, but a combination of several is reason enough to back up immediately.
SMART: every drive's internal diagnostic system
Almost every modern drive (HDD and SSD) includes a system called SMART (Self-Monitoring, Analysis and Reporting Technology) that logs dozens of internal health metrics. Most of these metrics are noise to an average user, but an analysis by Backblaze covering tens of thousands of drives in production identified five SMART attributes that, combined, predicted 76.7% of drive failures before they happened.
The five SMART attributes that actually matter
| SMART attribute | What it measures | Warning sign |
|---|---|---|
| SMART 5 | Reallocated sectors count | Any nonzero value; over 100 is a serious warning |
| SMART 187 | Reported uncorrectable errors | Any nonzero value |
| SMART 188 | Command timeout | Values increasing over time |
| SMART 197 | Current pending sector count | Any nonzero value |
| SMART 198 | Uncorrectable sector count | Any nonzero value |
An important limitation worth acknowledging
The same Backblaze study found that 23.3% of drives that failed showed no prior warning in these SMART attributes — meaning SMART predicts most failures, but not all of them. This means the absence of SMART warning signs isn't a guarantee that a drive is completely healthy; it's a useful early-detection tool, not a definitive proof of drive health.
What to do if you spot signs of failure
Back up critical content immediately, before doing anything else — don't wait to "see if it gets worse." If the drive still works, copy it to another device while you still can. Consider replacing the drive as soon as your backup is secured, instead of continuing to trust a drive that already showed signs of active wear. You can calculate how much space that backup will need with the backup growth calculator.
Verifiable sources
The analysis of which SMART attributes most reliably predict drive failure, and in what percentage of cases, comes from public research by Backblaze based on tens of thousands of drives monitored across its data centers.