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4-22] I'm finding corrupted files on the CD-ROMs I write




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This article is from the CD-Recordable FAQ, by Andy McFadden (fadden@fadden.com) with numerous contributions by others.

4-22] I'm finding corrupted files on the CD-ROMs I write

(2000/10/20)

There have been a fair number of people who have burned a CD-ROM only to
discover that, while they can read text files, run applications, and look
at graphics, they can't extract from .ZIP archives or run compressed
applications (e.g. some "Setup.EXE"s under Win95). A common complaint
is a dialog with "the file is not a valid win32 application".

The problem they're seeing isn't just corruption of .ZIP files though.
Most kinds of files have a lot of redundancy in them. If a single bit is
lost out of a long text file, the chances of it being noticed are very
slight. For an application, the chances of it causing a failure depend on
where in the file the error falls. For a compressed file, though, every
bit is significant, and in a .ZIP archive the CRC has a very high
probability of detecting errors. (CRC is cyclic redudancy check. Most
file archivers compute a 32-bit CRC on the uncompressed input and store it
in the archive. When you extract the files, the CRC is checked to ensure
that nothing has been damaged.)

Eliminating these errors could be as simple as replacing a bad SCSI cable.
One way to narrow the possibilities down is to try the disc in different
readers on different machines. If the same error shows up in the same
place, the error was introduced during writing rather than while reading
the data back. Another thing to try is to burn the same disc twice. If
the data written to the CD-Rs doesn't match the original, but they do match
each other, then the errors are happening in the same place every time,
rather than at random, so the trouble might be with a driver or firmware
instead of a flaky cable or bad RAM.

If a file appears to be getting corrupted on the CD-R, try copying it back
to the hard drive and then comparing it to the original. If possible, see
if the file is missing large chunks or just has sporadic damage
throughout. You can use the DOS "fc" command (e.g. "FC /B FILE1 FILE2")
or one of the fancier applications listed in section (3-22).

If you can identify the problem as being with the reader or the writer you
may be able to focus on just one part of your system. If the trouble
appears to be with your writer, and you can't get it to work, try to move
it to somebody else's system and see if it works from there. It's
possible, though unlikely, that the CD recorder is flaky.

Whatever the case, the place to start is to check all cables, connections,
SCSI termination, L2 cache, and RAM. One user with an otherwise properly
functioning system was able to fix the CD-R corruption problems by correcting
the RAM timings in the BIOS setup. Use a memory tester, such as "Memtest86"
from http://www.memtest86.com/, to look for bad RAM. A couple of others
found that their problems went away when they disabled the L2 cache on the
motherboard. Sometimes adding a new device will make cables (especially
longer ones) turn flaky. Sometimes the flakiness only affects one device.
Swapping the cables is inexpensive, easy, and very likely to root out
the cause of your problems. Section (4-17) has some tips on SCSI stuff.
If your problem is media compatibility, and the errors are a result of the
BLER (error rate) exceeding the error correction's ability to fix them,
then changing to a different brand of media might help.

One last thing: make sure the original files are valid before you go on a
wild goose chase!


 

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