AFAIK, the difference is in the distance between the read/write head and the platter. In a hard disk drive, the very slightest speck of dust getting between them can cause a disk crash. On a CD/DVD drive, the distance between the head and the platter is much greater, making them less susceptible to problems caused by contaminants. I believe that's why hard drives are sealed. I'm probably completely wrong, though.
Firstly they use totally different technologies, hard drives use magnetic technology and CD/DVDs use optical technology and have much less capacity. Hard drives are assembled in clean rooms with filtered air and operators in special clothing as the distance between the heads of a hard drive and the platters is so fine that smoke particles can cause a head crash, that is why they are sealed after manufacture.
CD/DVDs use a laser beam to scan the surface of the disc which is much more tollerant with regard to contamination and put up with finger marks etc.
Hope the above helps.
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