Sector reallocation after a power outage

[Wicks]

Hey guys!
I have a question for a HDD expert! Yesterday we had a small blackout, restarted the computer, everything works fine, but I always check S.M.A.R.T. after a blackout just to be safe. This time however I had 1 sector reallocated, and the SMART warned me about it. Today I ran HDTune and the Windows built-in chkdsk, but NO error, no bad sector was found. Is it still safe, or should i get a new one asap?

 

[Someone Somewhere]

You're saying that there's no errors? So... why are you worried?

As always, a good backup is essential, but I think you're being a little too paranoid.

 

[Wicks]

Ye, maybe, but if there are no bad sectors why it had to reallocate one, maybe bug :S

 

[Wicks]

So guys, if i don't have bad sectors (or at least 2 tests said that), and the performance of the HDD is the same, then it does not have any problem. At least for now.

 

[Paperdoc]

You do not have a HDD problem. You do have an anxiety problem, though, so maybe an explanation will help.

The SMART message you got about reallocating one Sector is the result of a self-diagnosis operation that occurs in the HDD unit itself. Once it is done, the OS (Windows) does NOT know anything about that. What happened is that, within the HDD unit, a read operation (which includes automatically an assessment of the signal quality from that Sector) decided that the signal was questionably poor. The process then re-read a few times to be sure it got correct data from that Sector (which it did succeed at), then wrote it to a new known-good Sector (it keeps its own stash of good spares), substituted this new one for the old one, and noted in its own on-disk system that the old one is bad and should never be used. This self-checking and -repairing system is normal in all modern HDD's. Each time it is done, the SMART system records that one Sector was replaced with a good one. Eventually with many repeats of the process, the stock of known-good spares will be reduced to the point that the SMART system will issue a waring that the number of successful replacements is getting high. When that happens it the time to plan a replacement of the HDD unit whilte the unit is still functioning properly, and before the self-repair process is no longer able to function. HOWEVER, this NOT your case, so don't worry now.

So, why does Windows not show you that in CHKDSK? It really does not know all that stuff the HDD does internally. When Windows requests data from the HDD, the only time it knows there is a problem is when the HDD FAILS to get a clean read from a Sector after using its internal self-checking process. When such a failure happens after numerous attempts to re-read a Sector, the HDD tells Windows it has failed. THEN Windows records a Bad Sector and uses its own processes to log that. (Unfortunately, when this happens the data could not be read successfully and so could not be recovered, and Windows is forced to assign a new good Sector to the file it was trying to use, BUT that new sector does NOT contain the original good data.) In your case, the self-diagnosis and -repair process internal to the HDD did NOT fail, and Windows got the good data it requested, with NO messages of failure. So Windows has no idea that a problem ever existed or was fixed "behind the scenes".