Re: Copying what I can from bit rotted disks?

["Michael J. Mahon" <mjmahon@aol.com>]

Jayson Smith wrote:
> Hi,
>
>      I have one disk in particular that has sadly become a victim of bit 
> rot. It's a flippy with various music programs on both sides. It loads the 
> Textalker software, and was part of a set of talking disks with lots of 
> public domain software on them. It is DOS 3.3. I have no idea who put 
> together this collection of disks, or I'd ask if anyone has a replacement, 
> as well as any other disks in the set I don't have. There were probably over 
> fifty of these disks. In any event, on this particular disk, both sides have 
> been victimized by bit rot, and some of the programs are not copyable. I 
> assume, once bit rot has occurred, there's virtually nothing that can be 
> done to try to bring those files back from the grave?

You don't mention whether you have tried reading this disk on at least
two different drives--sometimes poor margins on one drive cause read
errors that another drive can read fine.  Margins can be degraded by
many things, including dirty heads, out-of-tolerance speed, and even
poor alignment (rare).

Generally, a controller card will not affect drive margins, so changing
controller cards is less likely to make a difference.

If the disk(s) are still bad on two or more drives, then a disk verify
will let you know which sectors are unreadable.  If they (or a subset of
them) are the same on two drives, then they are likely bad on the disk.

Even then, the disk may be recoverable either completely (if the bad
sectors are unallocated) or partially (the files containing the bad
sectors will, of course, be incomplete).

If the errors are in the file system metadata, like the directory, then
the disk may still be recoverable by a "repair" program that will find
the track/sector lists and rebuild the directory.  Such repairs are best
done on a sector copy of the damaged disk, so that no writes are done to
the damaged disk at all.

Before getting deep into recovering a disk with errors, it's worth
trying a "persistent" copy program, since the sectors may read correctly
every once in a while.  ProSel's "Volume Copy" is such a copier--it will
try repeatedly to read a bad area and, upon failure, will give an option
to continue copying substituting a block of zeros for the bad block
(it's a ProDOS-based copier, so it deals in blocks, not sectors).

If nothing works, I'd be happy to try to recover the disk for you.

>                          If that's true, than 
> I'd love to find a program that would take the disks, and copy all the files 
> it can to another disk. Possibly retry a few times if an error occurs, but 
> if all its efforts are for naught, don't stop the process entirely, just go 
> on to the next file, and make a notation of which files could not be copied. 
> Obviously, FID will copy all files, but when it hits an I/O error, it stops. 
> I can manually copy each file, making a list of those that can't be copied, 
> but surely there must be a more automatic way. Something you can just set 
> and forget, and when you come back, it says, "I copied this disk, I could 
> not get the following files..." Any thoughts?

I'm sure there's a modification of COPYA that will do that, but I don't
have it myself.  The "driver" for COPYA is Applesoft, so you might take
a look at it and see if you can modify it to just print out the bad
filename and go on to the next file instead of quitting.

-michael

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