What do reallocated sectors mean on a NAS drive?

The Reallocated Sector Count SMART attribute records how many bad sectors the drive has detected and remapped to spare sectors. It is the drive quietly routing around physical defects.

How to read it

• Zero: ideal — no remapped sectors.
• A few, stable: common, especially as a drive ages, and not by itself an emergency. Note the number and watch it.
• Rising over time: the warning sign. A count that grows week to week means the drive surface is degrading and the drive is heading toward failure.

The trend matters more than the absolute number. Ten reallocated sectors that have not changed in a year is very different from ten that appeared this week.

What to do

1. Make sure you have a current backup — reallocated sectors mean the drive has already shown physical defects.
2. Check the trend: re-read the SMART value over days. Growing = replace.
3. Run an extended SMART test to surface further issues.
4. If the count climbs, or if Current Pending Sectors also appear, replace the drive with a CMR NAS drive.

In a RAID array a single suspect drive is not an immediate data-loss risk, but it erodes your redundancy margin, so replace proactively. The table lists CMR NAS drives by price per terabyte.