SSD health is not a single countdown. For an NVMe drive, read the current Critical Warning bits first, then compare Available Spare with its threshold, interpret Percentage Used as an endurance estimate, compare composite temperature with that drive’s own warning and critical limits, and check whether media-integrity errors are appearing. A value of 100 for Percentage Used means the manufacturer’s estimated endurance has been consumed; it does not, by itself, prove that the SSD has failed. Any critical warning, degraded reliability state, or read-only transition is a reason to protect important files promptly.
What “SSD health” actually measures
Utilities use the word health as a summary of several SMART or NVMe log fields. It is not a guarantee that the drive will keep working for a particular number of months, and it is not a laboratory pass/fail certificate. A health label can remain “good” while a drive has heavy usage, or change suddenly when the controller detects a threshold violation.
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Intel describes its own health result as being based on current SMART attributes and warns that its remaining-life estimate depends on the applied workload. If the workload changes, the estimate may no longer be accurate. That limitation applies broadly: endurance calculations are predictions based on the manufacturer’s model, not a calendar appointment for failure.
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The most consistent field definitions are available for NVMe drives. Older SATA SSDs expose vendor-specific SMART attributes, so the same raw number can mean different things on different models. For SATA, use the drive maker’s documentation or a tool that identifies the model-specific attribute names before drawing conclusions.
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First identify the drive and interface
Before interpreting any number, record the SSD’s exact model and determine whether it uses NVMe or SATA. The interface affects which fields exist and which Windows screens can read them.
- NVMe: usually an M.2 module or a PCIe add-in card, with a standardized NVMe SMART/Health Information log.
- SATA SSD: a 2.5-inch drive or SATA-connected M.2 device, with SMART attributes whose names and raw-value conventions vary by manufacturer.
Do not assume that an M.2 shape means NVMe; M.2 is a physical form factor, while SATA and NVMe are different interfaces. Check the model in your system’s storage details or the computer manufacturer’s specifications.
Checking health in Windows
On current Windows releases, the built-in view is intended for NVM (NVMe) SSDs. Open Settings > System > Storage > Advanced storage settings > Disks & volumes, select the disk, and choose Properties. Labels can vary slightly between Windows versions.
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The NVMe fields that matter most
| Field | What it represents | How to interpret it |
|---|---|---|
| Critical Warning | A bit field showing active warning states. | Any set bit needs investigation. Multiple bits can be set at once. |
| Available Spare | Normalized percentage of spare capacity remaining for replacing worn or failed NAND blocks. | Compare it with Available Spare Threshold. A value below the threshold is a warning, not ordinary free-space usage. |
| Percentage Used | Manufacturer-estimated percentage of the NVM subsystem’s endurance consumed. | It is an endurance estimate, not a remaining-years figure. 100 means estimated endurance has been consumed; values above 100 are permitted. |
| Composite Temperature | A temperature for the controller and NVM subsystem, reported by NVMe in kelvins. | Compare it with that model’s WCTEMP and CCTEMP values rather than a universal temperature rule. |
| Data Units Written | Host data written, excluding metadata, in 512-byte units reported in thousands and rounded up. | Use it as a workload counter and compare it with the SSD’s endurance specification. |
| Media Errors | Detected unrecovered data-integrity errors. | A nonzero or increasing value is more concerning when accompanied by warnings or reliability problems. |
| Error Information Log Entries | The number of entries recorded in the controller’s error-information log over its life. | Trend the count and inspect the associated errors; the count alone is not a failure date. |
| Unsafe Shutdowns | Power losses or resets that occurred without the required shutdown notification. | It explains past power events but does not, by itself, predict imminent SSD failure. |
How to read Critical Warning
Critical Warning is a bit field, so a utility may show a number, a set of checkboxes, or a sentence listing several conditions. Relevant NVMe flags include:
- Available spare below threshold: the reserve pool has fallen below the drive’s configured minimum.
- Temperature outside a threshold: the composite temperature is at or beyond a warning or critical limit defined by the device.
- Degraded reliability: the controller reports that reliability has deteriorated.
- Read-only media: the drive has entered a state in which data can be read but writes are unavailable.
- Volatile-memory backup failure, where supported: the controller’s power-loss protection backup has a reported problem.
The flags describe the controller’s current associated state; they are not a permanent history of every warning the drive has ever issued. A cleared flag therefore does not prove that no earlier event occurred. If degraded reliability or read-only media is reported, treat the files on the drive as at risk and follow the protection procedure in the warning-response section below.
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Percentage Used and “remaining life”
NVMe calls the underlying field Percentage Used. It is the vendor’s estimate of the percentage of the NVM subsystem’s life consumed, based on actual usage and the manufacturer’s endurance prediction. Some programs present an approximate “remaining life” percentage instead. Those labels are not interchangeable unless the program explains its calculation: 20% used is not necessarily the same display value as 20% life remaining.
A value of 100 indicates that the estimated endurance has been consumed, but Microsoft’s NVMe definition explicitly notes that this may not indicate an NVM subsystem failure. Values above 100 are allowed. The controller may update the estimate approximately once per power-on hour while it is awake, so the number can remain unchanged for a while and then move in a step.
Do not convert Percentage Used into “months left.” NAND wear depends on the amount and pattern of writes, write amplification, temperature, firmware behavior, and the manufacturer’s model. A workstation that changes from light office use to sustained video-cache writes can consume the estimate at a different rate. Use the figure to plan monitoring and replacement, not to postpone backups until it reaches zero.
Available Spare is not free storage space
Available Spare is a normalized percentage of reserve NAND capacity. The controller uses spare blocks to replace blocks that can no longer be used reliably. Available Spare Threshold is the drive’s own alarm level. A critical warning is generated when the spare percentage falls below that threshold.
This field has no relationship to the unused gigabytes shown in File Explorer. You can have hundreds of gigabytes free and still have low spare capacity, or a nearly full file system with normal spare capacity. Read both fields separately: maintain reasonable operating-system free space for performance, but use Available Spare only for the controller’s reserve-block status.
Interpreting SSD temperature correctly
NVMe’s composite temperature is reported in kelvins and covers the controller and NVM subsystem. It is a composite sensor value, not necessarily the temperature at one physical point on the module. If a utility displays kelvins, convert to Celsius by subtracting 273.15; for example, 323.15 K is 50 °C.
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Each drive exposes a WCTEMP (warning composite temperature) and CCTEMP (critical composite temperature) value. Those thresholds are the correct reference for that model. There is no single “safe SSD temperature” that applies to every NVMe device, because firmware, NAND type, cooling design, and the manufacturer’s limits differ.
Some health logs also report accumulated minutes spent above the warning and critical composite-temperature thresholds. A normal current reading with a large historical over-temperature counter tells a different story from a drive that is cool now and has never crossed its limits. Check airflow, heatsink contact, dust, and workload if the current value approaches WCTEMP or the historical counters are increasing.
What the write and error counters tell you
Data Units Written
Data Units Written counts host-written data in units of 512 bytes, excluding metadata. The value is reported in thousands and rounded up: one reported unit represents 1,000 512-byte units. It is therefore a usage counter, not a direct percentage. To assess endurance, compare the accumulated writes with the specific SSD’s published endurance rating and account for the workload that produced them.
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Media Errors
Media Errors counts detected unrecovered data-integrity errors. A count that remains stable can have a different significance from a count that increases during normal operation. Investigate promptly when new errors appear, especially if they coincide with a Critical Warning, degraded reliability, corrupted files, or system events.
Error Information Log Entries
This counter records entries in the controller’s error-information log over the drive’s life. Open the individual entries when the tool allows it; the total does not identify the cause or predict a failure date by itself.
Unsafe Shutdowns
Unsafe Shutdowns increments when power is lost without the required shutdown notification. A high number can reflect a history of outages, forced resets, or battery depletion rather than NAND wear. Reduce repeat events by fixing power, cabling, battery, or system-stability problems, then watch whether new media errors or warnings develop.
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A practical reading sequence
- Identify the model and interface. Record the exact model, firmware if shown, and whether the device is NVMe or SATA.
- Read Critical Warning first. Expand the field or decode the individual bits so you know whether the issue is spare capacity, temperature, reliability, read-only media, or another supported condition.
- Compare spare values. Put Available Spare beside Available Spare Threshold. A spare value below the threshold is a controller warning even when the file system has free space.
- Interpret Percentage Used. Treat it as a manufacturer-based endurance estimate. Do not turn it into a date, and do not treat 100 or a value above 100 as proof that the drive has already failed.
- Check temperature against device limits. Convert kelvins if necessary, then compare the current composite value with WCTEMP and CCTEMP. Review accumulated minutes above those limits when the log supplies them.
- Review counters and trends. Note Data Units Written, Media Errors, Error Information Log Entries, and Unsafe Shutdowns. Recheck after a known interval to see which values are changing.
- Act on the strongest signal. A current critical warning or reliability problem takes priority over a reassuring percentage label. Protect data and obtain model-specific support before continuing normal heavy workloads.
What to do when a warning appears
Critical warning, degraded reliability, or read-only media
Back up important files promptly and verify that the backup can be opened. Microsoft’s guidance recommends backing up when its critical storage warning appears. If the drive is read-only, existing data may still be readable even though new writes are unavailable; copy the most important files first, avoid unnecessary repair operations, and contact the computer or SSD manufacturer for the next steps.
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Record the current values and firmware, make a verified backup, and check the manufacturer’s documentation for the model’s spare threshold. Plan a replacement based on the warning and your risk tolerance rather than waiting for a displayed life estimate to reach zero.
High temperature or rising over-temperature minutes
Compare the reading with WCTEMP and CCTEMP before calling it abnormal. Then inspect the cooling path: confirm the heatsink pad contacts the module, clean blocked vents, verify that fans run, and check whether a sustained workload is causing the rise. Recheck the composite value and threshold counters after the physical issue is corrected.
Media errors or rapidly changing error logs
Prioritize a verified backup, capture the model and error details, and use the manufacturer’s diagnostic guidance. Do not infer a precise failure date from the counter. A changing error pattern is more actionable than an old, unchanged count.
Why SATA readings need different treatment
SATA SMART attributes are not standardized in the same way as the NVMe health log. Names such as “wear leveling,” “remaining life,” or “total LBAs written” can use different normalized values and raw encodings across vendors. A raw number copied from one SATA model cannot safely be compared with the same attribute on another model.
When a SATA drive is involved, use the exact model’s documentation or a compatible utility that explains its normalized and raw columns. Windows’ NVM-only health page cannot fill that gap. If the tool reports a generic health percentage without defining its inputs, treat it as a summary and inspect the underlying vendor-specific attributes.
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When replacement is justified
Replacement is justified by evidence of degraded reliability, read-only media, a persistent critical warning, worsening media errors, or a spare-capacity warning that the manufacturer identifies as a service condition. Percentage Used alone is a planning signal, not an automatic replacement command.
Choose a replacement only after checking the computer’s constraints:
- Interface: SATA and NVMe are not interchangeable without the required connector and controller support.
- Form factor: an M.2 slot may accept only a particular keying, length, or protocol.
- Capacity: confirm that the replacement meets the operating system, application, and migration requirements.
- Thermal design: high-performance NVMe drives may require a heatsink or adequate airflow.
- Workload: sustained writes, virtual machines, media caches, and ordinary client use place different endurance demands on a drive.
The available evidence cannot identify an exact replacement model for an unspecified computer. Check the system manual and the existing drive’s model before purchasing.
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- “100% life used means it failed.” No. It means the estimated endurance has been consumed; the drive may continue operating, but the estimate no longer provides a comfortable endurance margin.
- “The health page is missing, so the SSD is bad.” No. The Windows page is for NVM SSDs and does not cover SATA SSDs or hard drives.
- “Available Spare is the free space shown in Explorer.” No. It is reserve NAND capacity managed by the controller.
- “Any temperature below one universal number is safe.” No. Use the model’s WCTEMP and CCTEMP thresholds and its historical over-temperature counters.
- “Data Units Written predicts the failure date.” No. It records host writes; endurance interpretation requires the drive’s specifications and workload context.
- “A nonzero error counter proves imminent failure.” No. Inspect whether errors are unrecovered, whether the count is increasing, and whether current warnings or reliability symptoms accompany them.
A simple monitoring routine
For a healthy NVMe drive, save the model, firmware, Critical Warning, Available Spare, Percentage Used, temperature, and error-counter values in a note. Recheck after major workload changes, firmware updates, cooling changes, or unexplained crashes. Trending the values is more informative than comparing a single snapshot with an arbitrary internet benchmark.
Keep an independent backup regardless of the displayed health label. Health telemetry helps you prioritize maintenance; it cannot protect files after an unexpected controller, power, firmware, or system failure.
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