Hard disks have extensive facilities built into them to detect, correct and avoid data
errors; these are discussed in some detail here. Most
of the error-recovery procedures discussed in that section are reactive; they are
concerned with how to deal with an error situation that has occurred when data is
requested by the user. As suggested in the description of SMART, sometimes problems can occur due to gradual degradation, in this
case of a particular location on the media. If this is the case, it may possible to detect
a "bad spot" before it gets to the point where data loss might result. Since
hard disks use ECC to correct multiple-bit errors
but can only do so if the number of missing or incorrect bits is below a certain
threshold, detecting such errors quickly can prevent data loss that might occur if the
problem were not discovered until a later time.
To exploit this principle, some manufacturers are now incorporating routines to do proactive
error checking of the drive. During idle periods, the drive's logic board performs reads
over the surface of the disk to verify that the data can be read without errors. Any
errors encountered are typically treated the same way an error would be addressed during a
user read: the data may be rewritten to "refresh" it, or the sector where the
error is relocated may be remapped. Drives that
incorporate this feature usually integrate it with SMART; if an extensive number of errors occurs during such idle checks then
this can signal problems with the drive.
This is a useful reliability feature, but one that can sometimes cause confusion on the
part of hard disk users. The reason is that this scanning activity can cause the drive to
make "active" sounds even when the PC not doing anything involving the hard
disk; the hard disk activity LED will also not be
lit. (Since this scanning is purely an internal activity, it will not cause the hard disk
controller or motherboard to light the LED.) If your drive is one that incorporates this
feature and you see this "ghost" activity, there's nothing to be concerned
about. There also should be no impact on the performance of the drive, since the scanning
occurs only when the drive is idle.
Note: These scans
typically run only after the drive has been idle for some number of minutes. This means
you have to leave the drive on some time when you are not using it for them to activate.
If your normal operating mode is to turn on the PC, use it, and then turn it off, then the
drive may never become idle long enough for this idle-time scan to initiate.
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