hard drive data recovery

From: Allison J Parent <allisonp_at_world.std.com>
Date: Wed Jul 15 12:57:02 1998

< I'm not sure what that means, but I assume that the servo surface
< contains a sort of coordinate system that the servo head reads and thus
< guides the other heads. If that assumption is correct, why isn't it
< possible to realign the thing with another servo platter?

First it depends if the servo is embedded or dedicated platter or stepper
(or other absolute positioner).

For dedicated platter one head is assigned as the servo head and since they
are all mechanically one assembly the rest just follow along. Now if a
platter is changed it's data may be offset relative to the others. Same
thing is true for replacing a head as the RELATIVE alignment between heads
is disturbed. This case I believer the disk can be low level formatted
and reused but existant data may be lost. If servo information does not
exist (DeMIL'd drive) it's likely good for parts as servo tracks are hard
to regenerate.

If it's embedded servo per head each head as selected will follow the
selected track servo signals and making relative head alignment less
critical. If the servo is blown there is no way to regenerate easily.

For cases of absolute positioners like older ST506/412/225/251 (and a swarm
of others) drives getting alignment to cylinder 000 and beteeen heads would
be critical for complete data recovery. If the drive is to be reformatted
however a bulk erase and reformat will establish new tracks in some cases
and make a mess in others.

In the case of removable pack drives that is a totally different can of
worms though simpler as they so have alignment procedures for field or
depot use.

Allison
Received on Wed Jul 15 1998 - 12:57:02 BST