Understanding SMART Data and Drive Health
Every hard drive logs hundreds of internal statistics about its own health. SMART (Self-Monitoring, Analysis and Reporting Technology) exposes these values — but most monitoring tools just show a green or red icon. Here is what the numbers actually mean.
What Is SMART?
SMART is a monitoring system built into nearly every hard drive and SSD manufactured after 1996. The drive's firmware continuously tracks operational metrics — temperature, error counts, spin-up times, head positioning accuracy, and more — and stores them in an on-drive log that software can query over ATA or SCSI commands.
The problem is that SMART was never standardized beyond the basics. Each manufacturer implements their own set of attributes, thresholds, and what counts as a "failure." This makes raw SMART data difficult to interpret without understanding what each attribute actually measures.
The Attributes That Actually Predict Failure
Research by Backblaze (published from their data center fleet of 100,000+ drives) identified five SMART attributes that are strongly correlated with imminent drive failure. If any of these exceed zero on a healthy drive, take immediate action.
| Attribute ID | Name | What It Measures | Danger Level |
|---|---|---|---|
| 5 | Reallocated Sector Count | Number of sectors the drive has remapped due to read/write errors | Critical |
| 187 | Reported Uncorrectable Errors | Errors the drive could not fix via ECC — data may be permanently lost | Critical |
| 188 | Command Timeout | Commands that timed out — often indicates firmware or mechanical issues | High |
| 197 | Current Pending Sectors | Sectors flagged as unstable — waiting to be remapped on next write | Critical |
| 198 | Uncorrectable Sector Count | Sectors that failed reallocation — cannot be read or written reliably | Critical |
Other Important Attributes
Raw Read Error Rate
Seagate drives show large numbers here by design — it is a raw hardware counter, not a failure indicator for Seagate specifically.
Spin-Up Time
Time in milliseconds to reach full RPM. A value creeping upward over months can indicate bearing wear.
Power-On Hours
Total hours the drive has been running. Useful for judging the remaining life expectancy of a refurbished drive.
Temperature
Current drive temperature in Celsius. Keep drives below 45°C for maximum longevity. Above 55°C is a concern.
Power Cycle Count
Number of times the drive has been powered on and off. High counts on a desktop drive may indicate heavy use.
Airflow Temperature
An alternate temperature measurement used by some WD drives in place of attribute 194.
How to Read SMART Data
Each SMART attribute has three values: Raw (the actual hardware counter), Value (a normalized score, usually 1–253, where higher is better), and Threshold (the manufacturer-set minimum before the drive officially declares a failure).
Do Not Rely on "Threshold"
Manufacturers set SMART thresholds conservatively. A drive can be failing in practice while still showing all values above threshold. Focus on the raw counts for the five critical attributes above — any non-zero value for attributes 5, 187, 197, or 198 is actionable regardless of what the threshold says.
Recommended SMART Tools
CrystalDiskInfo
Windows
Free, simple, color-coded health status. Best tool for a quick health check on any Windows machine.
smartmontools
Linux / macOS
Command-line tool that exposes the full raw SMART dataset. The standard for NAS OS environments like TrueNAS.
Hard Disk Sentinel
Windows / Linux
Paid tool with drive temperature graphing, SMART history logging, and email alerting on threshold violations.
What To Do When SMART Shows Errors
Back up immediately
Do not wait. If attributes 5, 197, or 198 are non-zero, assume the drive can fail at any moment and get your data off it first.
Run a full surface scan
Use tools like badblocks (Linux) or CrystalDiskInfo's extended test to map the full extent of damage and identify which files are affected.
Do not RMA a drive with data
If the drive contains personal data, perform a secure erase before returning it. Manufacturers do not guarantee data destruction during RMA processing.
Replace, do not trust
A drive that has reallocated sectors once will reallocate more. Even if it appears stable, the failure curve accelerates. Replace it.
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