"David Candy" said in news:
[email protected]:
CRC errors mean the data on disk is not the same as what was written.
The data gets written and a number (calculated from the data) is also
written by the floppy hardware. When reading the hardware checks the
data against the number and raises an error if it doesn't match.
Computers won't use faulty data (a design decision) so everything
stops. Older versions of Nortons had Disk Tools which would read the
data and write it to a new file ignoring errors.
Your data may be ok and the number is corrupt. The number is
calculated in 512 byte sets so even if damaged most of your file is
ok.
Try coping from a command prompt and if you get a prompt that says
Ignore it will just ignore those 512 bytes. Word documents have a
text component and a binary component. Word itself has recover text
from any file convertor that will try to extract what text it can.
Yeah, I know what CRC means.
- Could be the preformatted floppy was created by faulty mass production
equipment that had its heads out of alignment. It has bappened and
means going back to the store to get a new batch won't help because it
is likely the entire batch the store has is bad (but you can try going
to another store). Or just reformat the floppy. The floppy drive used
to lay down the file may have been able to write okay but the floppy
used to read is out of alignment. Parts of the file may be readable,
other sectors fail, so the CRC won't match and the file looks bad so you
get that error.
- If you formatted the floppy, could be that drive is out of alignment
in one direction (but within specs) and the floppy drive you read from
is also out of alignment (but within specs) but the two misalignments
together exceed the width allowed and you get a read error which again
results in a CRC error.
- Formatting a drive lays down magnetic patterns whose dipoles are under
stress. Over time, and because the dipoles want to return to their
least stress position (which is parallel each other instead of in
alignment), the media goes "soft". You end up losing bits over time on
magnetic media. Since floppies get carried around in pockets, suffer
larger ranges of heat and physical stress, and so on, they go soft
faster than the rust on the hard platters of a fixed drive. When you
wrote the file it was okay because the dipoles got reinforced in their
position. Over time, some dipoles start to drift, the media goes soft,
and you lose bits (aka retentivity wanes). That causes a CRC error on a
read (performed a long time later). That's why some utilities will
actually read the data off a floppy, run through a couple patterns to
"wash" that spot on the floppy, and write the data back down in the same
place. It's called refreshing your floppy.
- A gamma ray hit a dipole and changed its position.
- The floppy got near a magnetic field during transport or storage. An
old telephone with the transformer for the ringer clapper, a magnet, a
magnetized screwdriver (don't use those around computers), or whatever.
The magnetic flux across the floppy corrupts the data and you get a CRC
error.
- I've seen some floppies that have a permanent "soft" spot where
retentivity is always weak (happens on hard drives, and even more so
with plated platters when the plated surface starts to bubble away from
the substrate). You might be able to write but reading is iffy. The
drive itself will retry something like 5 times to read a bad spot and
the OS will also attempt several retries. With several OS retries per
every hardware retry, the number of retries is something like 15 times,
so it could be a flaky spot that manages to eventually get read okay.
- The heads on the write or read drive are dirty, or the floppy is dirty
(the liner is supposed to help but obviously it can get polluted over
repeated uses and stop removing filth). Some bits read okay, others
don't, so you get a CRC error.
- The CRC is a hash code from the bits in the file. The data might be
okay but the CRC is wrong or got corrupted. I've seen this also happen
with WinZip when trying to store thousands of files in a .zip file; the
computed CRC that got stored in the .zip file was wrong, the data is
okay, so you have to tell Winzip to extract despite the CRC error. This
error was repeatable and the reason I had to abandon WinZip and go with
PKZip.
There are probably other causes. The point is that the problem is
hardware based. The above just points out some possible causes. If the
file still exists on the source computer, format the transport floppy on
the target computer (so it gets formatted on the drive that will read
the data), take it to the source computer to write the file (still using
the tracks laid down by the target drive), and then read it on the
target computer. Otherwise, time for some cleaning, refreshing,
reformatting, and possibly replacing a floppy drive. You can buy
testing software that includes a calibration floppy but it would be
cheaper just to replace the floppy drive.
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