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Windows’ “Healthy” label does not mean an SSD has no errors. It means the disk is functioning normally according to the limited status that Windows can read. NVMe telemetry can simultaneously show media errors, error-log entries, a low spare reserve or degraded reliability. To judge the drive, identify its interface and inspect the complete health record rather than relying on one headline or percentage.
Why Windows and SMART can appear to disagree
Microsoft defines a healthy disk status as “the disk is functioning normally.” That is a broad operating-status description, not a complete report of the controller’s NVMe health structure. The structure contains separate fields for critical warnings, available spare, percentage used, temperature, media errors and error-log entries.
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There is also a coverage limitation: Windows’ Storage health view monitors NVM/NVMe SSDs, but not SATA SSDs or hard-disk drives. A SATA drive may therefore show little or no useful information in that page even when a SMART-capable utility reports extensive attributes.
What the important NVMe fields actually mean
Critical Warning
This is a bit field, not a simple quality score. Bits can indicate that spare capacity is below its threshold, the device is overheating, NVM-subsystem reliability has degraded, the media has become read-only, or a volatile-memory backup has failed. Any asserted critical warning deserves immediate backup and investigation.
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Available Spare
Available Spare is a normalized 0–100% value for remaining spare capacity. The controller compares it with the drive’s own spare threshold. A value below that threshold is materially different from a merely high Percentage Used reading: it indicates the reserve the drive relies on for replacing worn or failed flash blocks has fallen too far.
Percentage Used
Percentage Used is a vendor-specific estimate of the percentage of NVM life consumed. It is calculated from actual use and the manufacturer’s prediction of NVM life. It is an endurance estimate, not a universal failure countdown. There is no authoritative, cross-model percentage at which every SSD fails, so applying a generic internet cutoff can produce a false diagnosis.
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Temperature
Temperature is the device’s composite temperature. Interpret it against the exact model’s specifications and the conditions under which it was recorded; a brief peak during a heavy transfer is not equivalent to sustained overheating.
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Media or data-integrity errors indicate problems reading or validating stored data. Error-log entries may record controller, command or transport events. A drive can still report a reassuring Percentage Used value while these counters are nonzero, so the counters and their trend matter more than a single life estimate.
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How to check an SSD without guessing
- Back up first. Copy irreplaceable files to another physical device or a verified cloud backup if you see any warning, accumulating errors, crashes, corruption or read-only behavior. If reliability is degraded, Microsoft’s guidance is to back up data and contact support.
- Identify the interface. Confirm whether the device is NVMe or SATA, then record its exact model, capacity and firmware version. Windows’ built-in health page is not a complete monitor for SATA SSDs or HDDs.
- Read the full telemetry. Use the SSD maker’s utility or a SMART-capable reader such as CrystalDiskInfo. Record Critical Warning, Available Spare and its threshold, Percentage Used, temperature, media/data-integrity errors, error-log count and whether the device is read-only.
- Check the model documentation. SMART names, raw-value formats and thresholds vary by vendor and firmware. Compare the recorded values with the exact model’s utility guide or datasheet rather than a generic table for a different SSD.
- Watch the trend. Recheck after normal use and note whether errors, temperature problems or spare loss are increasing. A stable, warning-free percentage is a monitoring signal; it is not proof that the rest of the device is error-free.
Which readings require replacement planning?
| Reading or behavior | What it suggests | Practical response |
|---|---|---|
| Critical Warning asserted | One or more controller-defined failure conditions is active. | Back up immediately, avoid treating the drive as dependable, and contact support or plan replacement. |
| Reliability degraded | Significant media-related errors or an internal NVM-subsystem error has been detected. | Preserve data first, then arrange replacement and investigate the vendor’s diagnostic result. |
| Available Spare below its threshold | The reserve used to replace worn flash is too low for the device’s design limit. | Back up and replace; do not wait for Percentage Used to reach a supposed universal number. |
| Read-only state | Data can still be accessed, but the device cannot accept writes. | Copy data immediately and replace the drive. |
| Media/data-integrity errors or rising error-log count | Read, validation, controller or interface faults may be occurring. | Back up, check cables/slots and firmware as appropriate, and replace if errors persist or accumulate. |
| Only an unusual Percentage Used value, with no warnings or errors | An endurance estimate that may be interpreted differently by the vendor. | Keep current backups, monitor the trend and consult the exact model documentation before replacing solely on that number. |
Why a drive can be “healthy” with media errors
The headline and the detailed counters answer different questions. “Healthy” describes whether the disk is currently functioning normally from the operating system’s perspective. Media errors describe events detected by the device while reading or validating data. If the controller has corrected an event, if the count is historical, or if the operating system’s health interface does not expose that attribute, both statements can be true at once.
The same distinction applies to endurance. Percentage Used estimates consumed flash life; it does not certify the controller, NAND integrity, temperature history, power-loss protection or the communication path. Treat the headline as one signal in a broader diagnosis.
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Compare two drives or two readings correctly
When evaluating an old and replacement SSD, or comparing before-and-after readings, keep the comparison like-for-like:
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- Interface: NVMe versus SATA.
- Exact model, capacity and firmware.
- Critical Warning state and the specific bits asserted.
- Available Spare and that model’s threshold.
- How the manufacturer defines Percentage Used.
- Media/data-integrity errors and error-log count, including whether they are increasing.
- Temperature and the workload or cooling conditions during measurement.
- Read-only behavior, warranty status and the monitoring tool’s ability to pass device data directly through the system.
What to do before installing a replacement
If replacement is indicated, verify the computer’s requirements before ordering: M.2 keying, supported PCIe generation, capacity limits, physical clearance, thermal solution and operating-system support. An “NVMe SSD replacement drive” search is only a starting phrase; compatibility must be checked against the system and the exact drive specification.
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If no warning is present, spare remains above its threshold, errors are absent or stable, and the only concern is Percentage Used, continued use with tested backups is reasonable. If warnings, degraded reliability, read-only behavior or accumulating errors appear, data preservation takes priority over further diagnostics.
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