tool for mapping away bad blocks on an external disk

Michael Powell nightrecon at
Sun Apr 17 20:02:08 UTC 2016

Warren Block wrote:

> On Sun, 17 Apr 2016, Michael Powell wrote:
>> Back in the day  (Fixed Direct Access SCSI-2 device) you could use the
>> controller BIOS to do what used to be known as a "low level format" of
>> SCSI drives in order to attempt to squeak a little more life from them.
>> And if new bad spots did not begin to pop again right away you might be
>> good to go for a while. If they did it indicated mass media failure that
>> would just avalanche. The old Adaptec 1540 and 2940 controllers come to
>> mind (Ctrl-A to enter the controller BIOS during POST).
>> Can't do this through a USB subsystem.
> Or to pretty much any drive any more.  They will accept the low-level
> format command, but return immediately.  Or that's what happened the
> last time I tried it, years ago.

Most of the time when low-level format was discussed wrt to IDE drives it 
was fake. All it really did was fill the drive with zeros. It is possible to 
low-level an IDE, but generally the manufacturers would not release the 
ability to the public. They could low-level a drive but they didn't want end 
users doing it; they would rather sell you another drive. A true low-level 
regenerates the remap zone in tandem.

It isn't that it isn't possible, per se, but that's it's so dangerous to 
actually try it with an IDE drive the mfr does not want the end user to be 
able to even try. Hence the 'fake' write zeros to all media and call it 
"low-level" as a marketing gimmick. When you low-level a SCSI drive it's the 
real thing.
> It's been a long time since hard drives gave any direct access to the
> media.  Everything goes through the controller on the drive now, which
> handles all that transparently.  So blocks that appeared bad might be
> already mapped out by the drive.

In IDE the remap zone is finite. As long as it's not full the end user won't 
see errors as the drive is handling this behind the scene. Once the remap 
zone becomes full, then the errors have no place to remap to, and you begin 
to see the dead sectors. Low-leveling a SCSI drive wiped everything and 
reinitialized the bad sector table to reflect the current status, which could 
be repeated. To do this with an IDE drive it really can only be done by the 
manufacturer, and they won't. It's "buy a new drive time."


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