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(Sorry if this is posted twice, E-mail program acting up)

Hi again,
When I access large amounts of data from a DVD or CD, the drive
makes a rapid clunking sound, almost like some hard drives make rapid
clicking sounds. I know this is normal because all my disk drives, including
the one in my Xbox make this noise. If optical disk drives are anything like
hard drives, I assume this noise is the laser rapidly jumping back and
forward. I am just wondering why it does this. When I tell my PC to copy an
entire DVD or CD to the hard drive, since it ultimately has to read all the
data on the disk, why doesn't the laser just follow the spiral track and
slowly move outwards, a bit like a record player needle would? I've also
noticed that it only "clunks" when it is changing the file that it is
reading, for example; if it is copying 20 files per second, it will make the
noise twice every second, but if it is copying very large files, it will
only make the noise once it is finished and moves onto the next one (if that
makes any sense). While I'm asking this, why does a hard drive do the same
thing?
Thanks for any answers,
a confused newbie!