Of everything a drive reports about itself, three counters describe actual media defects: reallocated sectors, current pending sectors and uncorrectable sectors. Most other values describe usage. These describe damage.
Reallocated sectors
When a drive decides a sector is no longer trustworthy, it retires it and substitutes one from a reserved spare pool. The reallocated count is the running total of those substitutions. It only ever increases, so a non-zero value means the drive has already lost usable spares — not that it is failing right now.
Current pending sectors
A pending sector is one the drive could not read reliably but has not yet replaced. It sits in limbo: a successful later write can clear it, or it can convert into a reallocated sector. Pending counts that persist across checks are more concerning than a single non-zero reading.
Uncorrectable sectors
These failed to read even with the drive's error correction applied, which means data was not recoverable from them. In a disposition context this is the most direct evidence that a drive should not carry a resale grade.
Trend beats snapshot
A single reading tells you where a drive is. Two readings tell you where it is going. A drive whose counts are unchanged across checks weeks apart is behaving very differently from one whose pending count is climbing, even if today's numbers look similar.
How many is too many?
There is no universal cutoff, and any tool claiming one is guessing about your buyers. Three conventions are common enough in the resale trade to be worth knowing before you set your own:
- Uncorrectable sectors — most resellers reject on any non-zero value, because data was already lost there and no later write brings it back.
- Current pending sectors — a count that survives a second check is commonly treated as failing, while one that clears after the drive rewrites the sector usually passes.
- Reallocated sectors — direction matters more than the value: a small count that is stable across checks weeks apart is widely accepted on used media, while one that is still climbing is not.
Where you draw your own accept and reject lines depends on your buyers, warranty terms and drive class. Whatever you choose, apply it consistently and record the raw values against the drive serial so a decision can be revisited.
Reading all three
- Boot the ITAD Tools iPXE toolkit.
- Select Drive Health Check.
- Record the three sector counters, plus power-on hours for context.
The check is read-only and never writes to the drive, so it is safe to run on media that has not yet been wiped or graded.
Two ways to run the Drive Health Check
Bootable toolkit. The ITAD Tools iPXE image turns any machine into a checking station, with no operating system installed and nothing to install on the machine. Free, no account needed.
Host agent. A Windows or Linux binary that reads the same SMART values on a machine that is already running, so you can capture drive health while a wipe is under way instead of making a separate pass. Requires a free account.
Both read SMART only. Neither writes to the drive, repairs it, or clears any counter.
Frequently asked questions
What is a reallocated sector?
A sector the drive found unreliable and replaced with one from its spare pool. The count is cumulative and does not decrease.
What is a current pending sector?
A sector the drive could not read reliably and has not yet remapped. Pending sectors can clear if a later write succeeds, or convert into reallocated sectors.
What is an uncorrectable sector?
A sector that failed to read even with error correction applied. It represents data the drive could not recover.
How many reallocated sectors is too many?
There is no universal cutoff. Most resellers reject any non-zero uncorrectable count, treat pending sectors that survive a second check as failing, and judge reallocated sectors by direction: a stable count is widely accepted on used media, a climbing one is not. Your own lines should reflect your buyers and warranty terms.
Is a non-zero reallocated count always disqualifying?
Not automatically. Buyers differ on tolerance. What matters more than a single reading is whether counts are stable or climbing across repeated checks.
How do I read these values on a machine with no OS?
Boot the ITAD Tools iPXE toolkit and run the Drive Health Check. It reads SMART data read-only with nothing installed.