[no subject]

It may be worthwhile to try tweaking the speed of the drive to see if
you can better
recover the data.

Regards,
Mike Willegal

On Nov 12, 12:13=A0pm, "Michael J. Mahon" <mjma...@aol.com> wrote:
> Zardo Zapp wrote:
> > dog_...@macgui.com (D Finnigan) wrote:
>
> >>I have a question about the error-checking mechanism used on the Disk ]=
[.
>
> >>As I understand it, checksums are used to verify that the data read fro=
m the
> >>disk are valid and error-free. However, would there be a way to skip th=
ese
> >>checks? Is there any software that does so?
>
> >>I ask because I have more than one disk which has some data and program=
s
> >>which can be read successfully, and some data which cannot. These disks=
 are
> >>not copy-protected. Would it be worth my time to make a disk image of a=
 disk
> >>with only partially good data? Is this even possible?
>
> > I worked with Sheppy a while back to get a version of ImageMaker that
> > can do this. It worked well for me.
>
> ProSel's volume copy will retry reading a bad sector/block several
> times, then ofer the choice to continue re-trying or ignore it (and
> write a block of zeroes).
>
> > Yes, it's worthwhile. The corruption may not be on an area of the disk
> > you care about, and so you can salvage the rest.
>
> > What would be nice, though, is to get a list of the damaged sectors so
> > you know where the trouble may be. So far, I don't have a good way to
> > do that.
>
> You can use Cpoy II Plus' "disk verify" functiion to get a list of
> "bad" sectors, and the "map files" function to find which files they
> lie within, if any.
>
> >>Also, would there ever be a circumstance where the disk has become dama=
ged
> >>in an area such that the Disk ][ cannot tell whether a bit is 1 or 0 an=
d
> >>thus refuses to read that disk?
>
> > The hardware isn't that sophisticated. It'll read a 1 or a 0 no matter
> > what - whether that's what supposed to be on the disk is anothe story.
> > The checksum is only 8 bits, so if the disk is so bad that you get
> > essentially random data, then on average 1 out of 256 reads the
> > checksum will still match. In other words, the checksum only does a
> > fair job of detecting data corruption.
>
> But in the more usual case of a few errors, it is quite effective.
>
> -michael
>
> NadaNet and AppleCrate II: parallel computing for Apple II computers!
> Home page:http://home.comcast.net/~mjmahon
>
> "The wastebasket is our most important design
> tool--and it's seriously underused."