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Idle Time Error Checking

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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