Use the drive’s SMART data to read these counters. Attribute 9 is commonly labeled Power-On Hours or Power-On Time, while attribute 4 is commonly Start/Stop Count. The labels and the unit behind each raw value are model-specific, so the exact drive model and its documentation must be checked before you convert a number or judge the drive’s condition.
What these two counters actually tell you
Power-on time and start-stop count are historical records maintained by a drive’s SMART system. They describe how the device has been used; they are not, by themselves, a pass/fail test.
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- Power-on time: the accumulated time that the drive has been in a powered-on state according to the drive’s own counter definition.
- Start/stop count: the number of spindle start and stop events recorded by the drive.
- Power-cycle count: a separate record of power-on and power-off cycles when the drive exposes one. It is not interchangeable with start/stop count.
In common ATA SMART output, attribute ID 9 is associated with power-on hours or power-on time, and ID 4 with start/stop count. Those numbers are common conventions, not guarantees that apply identically to every model. Manufacturers can choose different labels, units, scaling and encodings. Treat the model’s documentation as the authority.
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Record the complete model number printed on the drive or shown by the operating system. “1 TB hard drive” is not enough: two drives with the same capacity can store attribute 9 in different units or formats.
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Record the interface and access path
- Note whether the device is connected through SATA, USB, a USB-to-SATA bridge, or a RAID controller.
- Record the operating system and the device path it exposes.
- For a bare SATA drive, verify that the connection supplies both data and power. A data connector alone cannot make an unpowered drive visible.
This information matters because a utility can only report SMART data that the controller or bridge passes through. A USB enclosure, RAID controller or operating-system-specific device path may require a different access selector than a directly attached ATA disk.
Read the SMART attribute table with smartctl
The smartmontools utility can display the SMART data exposed by an accessible drive. On a system that presents an ATA disk as /dev/sdX, a common command pattern is:
smartctl -A /dev/sdXReplace /dev/sdX with the actual device path. The command is an example pattern, not a report of a particular test. USB bridges, RAID controllers and some operating systems need a different device selector or access method; consult the utility’s documentation for that hardware.
What the output normally contains
The vendor-specific attribute table commonly includes columns similar to these:
| Column | What it represents | How to use it |
|---|---|---|
| ID | The attribute identifier, such as 9 or 4. | Use it to locate the commonly associated counters, then confirm the model’s definition. |
| ATTRIBUTE_NAME | The label supplied by the drive or the utility’s model database. | Read the label together with the model number; labels are not universal standards. |
| VALUE | A normalized score maintained by the drive. | Do not read this as a number of hours or cycles. |
| WORST | The lowest normalized value recorded for that attribute. | It is not a historical hour total. |
| THRESH | The drive’s comparison threshold for the normalized value. | It is meaningful only in that drive’s SMART scheme. |
| TYPE | Whether the attribute is classified as, for example, pre-failure or old-age by the device. | Use it as context, not as a diagnosis by itself. |
| UPDATED | When the attribute is updated, according to the device’s reporting conventions. | It does not change the raw unit. |
| WHEN_FAILED | A failure indication, if the drive has recorded one. | A blank field is not proof that the disk is healthy. |
| RAW_VALUE | The vendor-defined stored value. | This is the field that may contain time, cycles, a packed value or another encoding. |
The normalized VALUE and WORST fields are not substitutes for RAW_VALUE. SMART attribute fields are vendor-specific, and smartctl reports values and thresholds stored by the device rather than calculating what the manufacturer intended them to mean.
Find power-on time (attribute 9)
Look for ID 9 in the attribute table. It may be labeled “Power-On Hours,” “Power-On Time,” or something similar. Then inspect the raw value and verify its unit for the exact model.
Why the raw number may not be hours
Depending on the manufacturer, attribute 9 can be stored as hours, minutes or seconds. Some drives use a packed or otherwise special encoding. A raw value of 12,000 therefore cannot safely be converted to “12,000 hours” without model-specific evidence.
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The smartmontools drive database contains model-specific formatting rules, and smartctl documents formatting options for attribute 9, including minute-based interpretations. That is why a plain integer should not automatically be treated as an hour count. If neither the manufacturer’s documentation nor a matching model entry explains the format, report the displayed label and raw value without converting it.
How to state a value responsibly
Use wording such as: “The drive reports attribute 9 with a raw value of 12,000; the model documentation identifies the unit as minutes.” If the unit is not established, say: “Attribute 9 raw value: 12,000; unit and encoding not confirmed.” This preserves the evidence instead of presenting an uncertain conversion as fact.
Find start-stop count (attribute 4)
Locate ID 4, usually labeled “Start/Stop Count.” The conventional meaning is a tally of spindle start and stop cycles. It reflects mechanical spin-up and spin-down events, not necessarily the number of times the computer was switched on.
A system can remain powered while the drive’s spindle starts and stops because of idle-time power management. Conversely, a power event does not always map one-for-one to a spindle event. The drive’s own definition controls what is counted.
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Do not substitute power cycles for start/stop cycles
Some devices expose ID 12, commonly called “Power On/Off Cycles,” separately from ID 9. A product manual for a specific BarraCuda SATA SSD family, for example, lists separate definitions for power-on hours and power-on/off cycles. That example illustrates product-specific naming; SSD definitions should not be generalized to all hard-disk drives. When comparing devices, record each available counter under its exact label.
Illustrative output: which number to read
A typical attribute row might contain several numeric fields:
ID ATTRIBUTE_NAME VALUE WORST THRESH TYPE RAW_VALUE
9 Power_On_Hours 98 98 0 Old_age 12000
4 Start_Stop_Count 99 99 0 Old_age 1840This is an illustration of column placement, not output from a tested drive. The figures cannot be interpreted until the model’s documentation establishes the units and encoding. The raw values in the example are not automatically 12,000 hours and 1,840 starts.
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If a model-specific definition confirms that attribute 9 is in hours, the raw value can be reported in hours. If it confirms minutes, divide by 60 only when a fractional-hour result is useful and keep the original raw value in your notes. Never replace the original value with a conversion that hides the unit assumption.
How to interpret the counters without declaring a drive healthy or failing
Neither counter has a universal safe or dangerous cutoff. The available documentation does not establish one hours limit or one start-stop limit that applies to every manufacturer and model.
Use the complete SMART picture
Assess the counters alongside:
- the drive’s overall SMART health status;
- any attribute marked as failing or approaching its device-defined threshold;
- error logs and other relevant error attributes;
- the drive’s current ability to read and write data reliably; and
- a current, verified backup of anything you cannot replace.
A high accumulated-use value can be consistent with normal service, while a drive with a low count can still have defects, damage or a history that SMART does not reveal. These counters describe recorded use, not every aspect of condition, and they cannot certify that a disk is genuine, complete or safe for important data.
What the normalized threshold does and does not mean
A normalized value and its threshold are manufacturer-defined. A threshold of zero, a value near 100, or a change in the “WORST” field cannot be compared directly with another model’s numbers. The threshold is used within that drive’s own SMART logic; it is not a universal lifespan scale.
Checking a used drive before you buy or repurpose it
Power-on time and start-stop count can contribute to a history assessment, but they are only two records. Ask for the complete SMART table, the exact model number and the date of the reading rather than accepting a screenshot of two counters.
Comparison checklist
- Confirm that both drives’ attribute 9 units and encodings are known.
- Compare accumulated power-on time only after those units are confirmed.
- Compare start-stop count under the model-specific definition of a start or stop event.
- Record power-cycle count separately if the drives expose it.
- Check overall SMART status, failure indicators, error logs and relevant error attributes.
- Consider the limits of the source data: a counter can be reset, misreported through a bridge, or unable to describe events outside the drive’s own logging.
Do not call a used drive “healthy” solely because both counters are small, and do not reject one solely because the counts are large. The evidence supports a usage-history statement, not a guaranteed future-life estimate.
When the drive is not visible to smartctl
An inaccessible device is an access-path problem before it is a SMART-interpretation problem. Work through the connection in this order:
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- Verify visibility in the operating system. If the disk is not detected at all, check data and power connections, the enclosure and the device path before running SMART commands.
- Identify the actual path. A directly attached ATA disk, USB bridge and RAID-controlled disk may be exposed differently. Use the path and device selector appropriate to that controller.
- Check SMART pass-through. Some bridges or controllers do not expose the drive’s SMART pages, so a successful connection may still return incomplete information.
- Confirm the model. If the model string is unavailable or generic, do not apply a formatting rule from another drive.
- Preserve the raw report. Save the complete output, including model and interface details, before trying another access method.
Different output or a missing attribute table does not prove that the drive has no counters. It may mean that the current controller path does not pass them through.
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Only when the bare SATA drive is not otherwise accessible. If the drive is already connected and visible to the operating system through a compatible controller, a separate adapter is generally unnecessary.
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Treat this hardware as an optional setup solution, not as part of the SMART interpretation itself. Once the device is exposed correctly, the same model-specific rules for attributes 9 and 4 still apply.
How to document a reading for someone else
A useful report lets another person reproduce the interpretation without guessing. Include:
- the complete manufacturer model number and firmware identifier, if shown;
- the interface and controller path, such as direct SATA or a USB bridge;
- the operating system and smartctl version, when available;
- the exact command or access method used;
- the full attribute table, not only two copied numbers;
- the raw, normalized, worst and threshold fields for IDs 9 and 4;
- the source that establishes the unit or encoding; and
- the date of the reading and the drive’s overall SMART status.
If the raw value is unfamiliar, provide the model number and complete SMART table before asking for an interpretation. Without them, converting the number would be speculation.
Common mistakes to avoid
Reading VALUE as hours
The normalized VALUE column is not the accumulated time. Read the raw field and the model-specific definition instead.
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Assuming ID numbers are universal
ID 9 and ID 4 are common associations, not a promise that every drive uses the same labels or meanings. Verify both the label and the format.
Converting every raw value to hours
Attribute 9 may be expressed in minutes, seconds or a packed representation. Leave an unconfirmed value unconverted.
Confusing starts with power cycles
Spindle start/stop events, power-on time and power-on/off cycles are related but distinct. Record each counter separately.
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No universal cutoff is established for these counters. Examine overall SMART health, failing attributes and error information instead.
Calling a low-count drive safe
Low recorded use does not rule out current faults, damage, bad sectors or incomplete history. Maintain a backup and evaluate the complete report.
A compact workflow
- Write down the exact drive model and interface.
- Expose the device through a controller or bridge that passes SMART data.
- Run a compatible smartctl attribute command, such as
smartctl -A /dev/sdXwith the correct device path. - Locate IDs 9 and 4, while retaining the complete table.
- Read the raw values, not just normalized VALUE or WORST.
- Confirm each unit and encoding in the manufacturer’s model documentation or a matching smartmontools drive-database entry.
- Keep the value unconverted if its format is not established.
- Interpret the history with overall SMART status, error data and a current backup.
The result should be a precise statement of what the drive reports and what remains unknown, rather than an unsupported estimate of remaining life.
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