The CD laser is most probably knackered (to use the technical term). The
other disk types are all detectable and readable with the DVD laser. CD-Rs
cannot reflect the red/orange DVD laser. The chances are that if the disc
was detected, it wouldn't write a reliable image to it. You may still be
able to write to CD-RWs because they are written a completely different
way
and use a lower write power from the CD-laser.
Sadly, solid state lasers decay with use, the rate varying between
different
samples. Laser failure is the most common mode of failure for DVD/CD
burners (though the DVD laser usually fails first - but this would depend
on
use).
How could a failing laser head not read CD-Rs but can read CD-RWs and
CDs? These three type of disks all should use the same lens/laser.
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This is not the case. CD-RWs and prematered CDs can both be read with the
DVD laser and most DVD/CD drives will do precisely this*. In fact there is
no difference beween these and their DVD equivalents apart from the data
density. It is only if the drive cannot read the inserted disc with the DVD
laser or it is called upon to burn a CD-R or CD-RW that it will turn to the
CD laser for the job. CD-R is the only media which will not reflect the DVD
laser to any significant extent, and once the drive fails to detect the
media with the DVD laser will it turn to the CD laser and try with that.
*In fact early DVD video players only had a DVD laser and so could not read
CD-R disks even though they read premastered CDs without any problem. It
came as a surprise to many people that they could often read CD-RW discs,
but the low reflectivity made results variable. Modern DVD video players
have a CD laser solely for CD-R reproduction.
Bluray and HD-DVD video players have the same problem and have to be
equipped with a DVD laser solely for DVD-R discs and a CD laser solely for
CD-R disks. All the other disks can be read with the blue laser - and the
Panasonic Blu-ray player indeed does so.