[RndTbl] Open Source Backup/cloning to USB HDD

Dan Martin ummar143 at gmail.com
Tue Oct 27 18:37:14 CDT 2009

I have used dd for copying drives or partitions in the past, in order to
have a clone ready to swap in in the event of a failure.  I found it
extremely useful, much faster than some cheap commercial software.

One problem I have encountered is IO errors, especially when copying large
volumes.  The 'noerr' option allows the copy to complete, but of course does
not correct the problem.  I have had one or two errors on every second

For my Mac, I have some commercial software that can find the error and what
file (if any) it affects.  It can report the error to the HSF+ file system,
so that no other files use that sector.

I can't find any material that explains how this works -- what I surmise is
that the drive firmware hides defective blocks.  Obviously it does not hide
them all, or at least not instantly - so the file system tracks bad blocks
that the firmware has missed in order to avoid using them in files.  Using
dd circumvents the file system, so there is a risk of a corrupt file on the
target drive/partition.

The drives seem OK in every other respect, and software that monitors the
status of the drives does not indicate any impending failure.  I am
guessing, based on a small sample, that this is a 'normal' condition.

On Tue, Oct 27, 2009 at 3:36 PM, John Lange <john at johnlange.ca> wrote:

> Just out of curiosity, what is it that these various programs that
> people have recommended do with regard to imaging that "dd" doesn't?
> I've never looked at any of these in any great detail but so far as I
> can see they all add complexity while reducing functionality.
> Image a disk? dd
> Backup a file system? rsync
> to tape? tar
> to remote? ssh (or scp)
> scheduled? put any of the above in cron
> That's all you need. What am I missing?
> Regards,
> --
> John Lange
> http://www.johnlange.ca
> On Tue, 2009-10-27 at 15:26 -0500, Montana Quiring wrote:
> > This is from Clonezilla page:
> > Based on Partimage, ntfsclone, partclone, and dd to clone partition.
> > However, clonezilla, containing some other programs, can save and
> > restore not only partitions, but also a whole disk.
> >
> > Thanks for the feedback everyone!
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Dan Martin, MD
GP Hospital Practitioner
Computer Scientist
ummar143 at shaw.ca
(204) 831-1746
answering machine always on
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