Cyclic Redundancy Check: what is it, and how do I get rid of it on my newly burnt CDs and DVDs?

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 drives.

• 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 media properly.

• 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.

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 another drive.

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.