I'm sorry, but the accepted answer is wrong (well, its partially right, but not totally)
CD's/DVD's are slow and have horrible latency - while some of this may be fixable with the effort put into hard drives, you still have multiple drawbacks - most critically would be the very limited number of rewrites you can do on a CD/DVD/Blu Ray - the number of rewrite cycles are way lower then spinning disks. (You will also have technology issues to overcome which have already been solved for hard drives - like size requirements for multiple platters, difficulties with multiple lasers etc)
Yes and no. To the extent this is possible it already exists and is obsoleted. The Iomega Jazz drive did pretty much what you are talking about, but (I'm using informed speculation here) - because it was removable and needed to be read in multiple machines , the densities could not continue to increase because the environment could not be well enough controlled. The environment in a hard drive is exceedingly tightly controlled, including disk alignments, what parts of the disk are unuseable etc - with the specific platters characteristics being mapped out in the drives logic. Also, hard drive platters are intolerant of any particles on the platter - a particle 1/1000'th of the width of a hair - and way, way smaller then a fingerprint will render the drive useless.
Also, because of the size of a spinning platter, you are not going to get it much smaller then a laptop drive - ie the size of the platter - which you have anyway - defines the size of drive.