When I burn a CD or DVD, I frequently get a “Cyclic Redundancy Check” error
when I go to read it. The media has no scratches or anything obviously wrong,
so why is this happening? And more importantly, what do I do?
A “Cyclic Redundancy Check”, or more commonly just “CRC”, is an error
detection mechanism that makes sure that the data you’re trying to read from
media, such as hard disks, CDs and DVDs, is actually correct. By “correct”, we
mean that the data you read is in fact the data that was written.
When a CRC check fails, there are several possibilities and places to
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A CRC works by taking a block of the data that is about to be written to the
media, calculating a checksum (basically some math involving all the data, that
results in a number), and writing that number, along with the original data, to
the media. When the data is later read, that same calculation happens, and if a
different number results, then an error is declared – the data read was not the
same as the data written.
CRC checks happen each time you read the media. Many CD and DVD burning
programs will include a read pass immediately after writing, to ensure that the
data was written properly.
There are several ways that a checksum error could happen:
A Bad Write: if the device that wrote the media had a
problem while writing, it could have written the wrong data.
Dirt in the Writer: dust or other obstructions in a CD or
DVD writer can interfere with the laser and cause the bad data to be written.
Bad Media: poor quality media, particularly CDs and DVDs,
can sometimes “not take” the data that’s written to them. Perhaps there’s a
flaw in the physical media. These types of flaws may not be visible to the
naked eye – even a one-bit error can cause a CRC calculation to fail.
Bad Write Alignment: it’s fairly obvious that on CDs and
DVDs data is written in a circle on the media. However, exactly where that
circle lands is dependent on the alignment of the drive. It could be slightly
off-center, or skewed in some way. This is frequently the case if a CD or DVD
reads perfectly on the drive that wrote it, but fails when read on other
Scratches and Other Damage: you’ve mentioned that your
media’s not scratched, so this may not apply to you, but CRC checking is most
commonly thought of as a way to detect errors that result from physical damage
to the media after it’s been written. And once again, remember that a tiny
scratch, if in the wrong place, can do damage. If your problem is with multiple
CDs or DVDs and you’ve been handling them properly, then it’s unlikely that
this is the case.
Dirt in the Reader: much like dirt in the writer, dust and
other particles can interfere with a CD or DVD reader’s ability to read the
Bad Read Alignment: again, much like bad write alignment,
if the reader isn’t tracking to the same “circle”, it may not be able to read
the data. Some drives are better at compensating for this than others.
Bad Reading Drive: finally, it’s always possible that the
CD or DVD drive itself is having a problem reading in general.
try reading it on different drives.”
As you can see, there are lots of possibilities.
If the problem “travels” with the CD or DVD you’ve written … meaning that
it fails when read in several different devices, then the problem is most
likely with the writer or the media itself.
If the problem happens only when read on one specific reader, then that
reader, and not the writer or media, is most suspect.
Make sure that you’re using high quality, brand name blank CDs and DVDs.
Occasionally clean the inside of your computer, including carefully vacuuming
the CD or DVD tray from the outside to remove excess dust. If there is an
alignment problem, there’s little you can do yourself – you’ll need to use
Can bad data be “fixed”? – in a nutshell, no. I’ve actually
oversimplified the role of a CRC above. It typically includes both error
detection and correction. That means that the calculated checksum can
often also be used to determine what data is wrong, and return the correct data
instead, on the fly. This probably happens often, and you’d never notice. By
the time a CRC error has been declared, enough errors have occurred that the
error correction has failed. At that point, you’re pretty much out of luck.
If you have a CD or DVD that is reporting a CRC error, try reading it on
different drives. As I mentioned above, the ability of drives to compensate for
things like alignment problems, or “weak” writes, varies, and what’s unreadable
on one might be readable on another. If you do find a drive that
works, copy the data off immediately, and plan on burning it to a new CD or DVD
on a writer that’s known to be good.