what are the differences between dual cd writer and normal cd writer?

Question

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Answer 1

Do you mean to ask about "DVD" writers, not CD?

CD writers only work with CD-R and CD-RW media. Regular DVD writers only work with DVD+R/RW (plus) or DVD-R/RW (minus). Dual dvd writers work with both plus and minus media, and newer ones can also support DL (Dual or Double layer discs).

The following site may help explain:
http://www.videohelp.com/dvd

Answer 2

I think u mean DVD rather thann dual cd writer.
Dual DVD writers in both of surface of media. Which is unusable in most DVDs/CDs. But i think there are no CD dual writers are avalible.