Bad sectors: how bad can it be

David N davidn04 at
Wed Oct 28 20:12:47 UTC 2009

2009/10/29 Michaël Grünewald <michaelgrunewald at>:
> Polytropon wrote:
>> On Tue, 27 Oct 2009 21:05:39 +0100, Michaël Grünewald
>> <michaelgrunewald at> wrote:
>>> I have however a question: How do I verify that a hard-drive is
>>> accurately working if its firmware will hide the bad sectors as long as
>>> possible?
>> I think the smartctl program from ports/smartmontools is a good tool for
>> such verification. As far as I understood,
>> it can read internal error logs from the firmware.
> Hi, following your suggestion I used smartmon to get access to the SMART
> data. I have run an extended offline test (with-t offline I think). The test
> reported no error (!) and the bad sectors are now read/writeable (!!). Is it
> safe to think the problem is gone?
> # smartctl -l selftest /dev/ad10
> SMART Self-test log structure revision number 1
> Num  Test_Description    Status                  Remaining LifeTime(hours)
>  LBA_of_first_error
> # 1  Extended offline    Completed without error       00%       458     -
> # 2  Extended offline    Aborted by host               70%       456
> --
> Best regards,
> Michaël
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I've had this problem before on consumer grade HDD.
* The drive tries to read/write to a sector, it can't, then marks the
sector as bad and preps for remapping
* Remapping may take upto 10 minutes on consumer grade HDD, enterprise
ones usually remap within seconds. so this 10 minute lagg time will
timeout the read/write of the OS.
* Usually a remap is done on reboot or when its done internally and
data is copied (if it can).

If your smartctl says it has used up a spare block
(Reallocated_Sector_Ct), replace the drive ASAP. The drives will tend
to get more and more bad blocks after the the first one is found,
usually because the head is damaging the disks or the head itself is
damaged, or other reasons. If its under warranty they usually replace
is, talk to the manufacturer before hand.

David N

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