This article is from the CD-Recordable FAQ, by Andy McFadden (email@example.com) with numerous contributions by others.
When people talk about "C2 errors" they are usually referring to the rate
of uncorrectable errors found on a CD. For an overview of error correction,
see section (2-17). For a more detailed look, see section (2-43-3). These
values are returned by "surface scan" tools.
There are two flavors of C2 errors, and not all drives are capable of
reporting both. Uncorrectable C2 errors indicate data that has been lost.
On an audio CD the missing sound samples will be smoothed over, and on
a CD-ROM the errors may be corrected by an additional level of error
correction, so the flaws may not be noticeable. Correctable C2 errors
indicate data that is whole but will be lost if the disc degrades any
futher. Some applications now differentiate between the two by referring
to uncorrectable C2 as "CU error".
The fewer errors of either kind, the better. The results you get are the
combination of the writer and the media, and in some cases may be influenced
by the quality of the device used to read the CD. If performing the same
set of operations on two different brands of discs results in consistently
lower error rates on one brand than the other, you will probably be better
off with the lower-error-rate brand. It is entirely possible that a
different writer would yield the opposite results, so it's not reasonable
to say that brand X is better than brand Y without performing a rigorous
test with a variety of different recorders.
Some discs are poorly constructed, and may deteriorate faster than others.
For long-term archiving, it may be useful to re-examine discs periodically,
especially if you buy "cheap" discs in bulk. Having fewer errors today
means little if the disc is unreadable in six weeks.
Performing these tests on a disc recorded with track-at-once recording or
packet writing can result in unexpectedly high error counts, because the
gaps between tracks and packets look like damaged areas.
For drives capable of reporting the errors, you can use Nero CD Speed
(http://www.cdspeed2000.com/) to evaluate the error rate. For a more
thorough examination, you can buy "CD Inspector", which comes with software
and a slightly modified CD-ROM drive