When scanning CDs, you'll get numbers for C1, C2 and CU. A properly recorded
CD does not show any C2 errors, and especially not any CU errors. If you are
getting C2/CU errors on pressed audio cds, then either the disc is heavily
damaged due to scratches or finger prints, or the CD in question is intentionally
damaged by the music industry (they call it "CactusData Shield 200" and claim
that it were a protection against copying).
A disc which has been intentionally damaged by the music industry can look like this:
This is an 800 MB CD-R, recorded quite some time ago:
If a recorded CD-R looks like this, you should make a backup and throw the
old disc away.
Scan speed
Depending on the drive you are using and the exact type of the CD, you can perform scans
at speeds up to 48x. However, some people claim that higher scan speeds lead to an inaccurate
result.
This is simply wrong. The result you receive tells how much work the error correction
had to do when reading the disc. That means, if the result is good even at higher speeds, the
disc is good. If higher scan speeds lead to severely increasing error rates, the drive has
trouble with the disc in question. The following 3 pictures show a Taiyo Yuden CD-R, recorded
at 40x speed in an Asus DRW-1608P, scanned at 4x, 24x and 48x:
