[ale] semi [OT] help recovering data from memory card

[Michael H. Warfield]

On Wed, 2011-10-19 at 09:21 -0400, David Tomaschik wrote: 
> On Tue, Oct 18, 2011 at 9:37 PM, Michael H. Warfield <mhw at wittsend.com> wrote:
> > On Tue, 2011-10-18 at 08:51 -0700, Boris Borisov wrote:
> >> I'm going to follow this post carefully because I have to deal with
> >> the same problem :) missing partition information. Actually dmesg
> >> gives me message : sd 1:0:0:0: [sda] Assuming drive cache: write
> >> through
> >> sda: unknown partition table
> >> What I am doing now is dd if=/dev/sda of=usb8gb.img just in case. Will
> >> take a while with this USB.1.1 that I plugged into it :)
> >
> > If you get a hard failure, you may want to try dd-rescue on it.  That
> > will work around "holes" in the data and continue to recover data.  It's
> > what I use for hard drives, though I don't know how well it will work on
> > USB flash drives.

> I'm not sure whether or not dd-rescue has any benefits on flash media.
>  I would think that blocks on flash media are either functional or
> dead.  (The idea on dd-rescue being that the disk might be able to
> read it on a retried pass.)

> Adding the conv=noerror option to dd, however, should have no downside
> and has the upside that blocks AFTER the unreadable block will be
> read.

The main advantage, in this case, that I see, is that it uses a divide
and conquer methodology where it tries to read large blocks and, if it
gets an error, it divides down into smaller blocks and tries to divide
into large blocks of errors looking for smaller islands of working
blocks.  I suppose that dd with conv=noerror and a minimum block size
would probably work as well.  I wasn't considering the retry option
there although one probably shouldn't discount thermal problems.  I've
had some flash drives that would sometimes read and then sometimes not.

> -- 
> David Tomaschik, RHCE, LPIC-1
> System Administrator/Open Source Advocate
> OpenPGP: 0x5DEA789B
> http://systemoverlord.com
> david at systemoverlord.com

Regards,
Mike
-- 
Michael H. Warfield (AI4NB) | (770) 985-6132 |  mhw at WittsEnd.com
   /\/\|=mhw=|\/\/          | (678) 463-0932 |  http://www.wittsend.com/mhw/
   NIC whois: MHW9          | An optimist believes we live in the best of all
 PGP Key: 0x674627FF        | possible worlds.  A pessimist is sure of it!
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