In CD writing is done from innerside to outer side.. how is data written into hard disk?
I think hard drives store data across the hard drive with no concern to storing from center to edge. If your using XP go to disk defragmentation and you will most likely see files or part of files stored across the disk. If files were stored sequentially every time a file like a word processed document is added too, the new part would have to be written elsewhere on disk making it inefficient. The disk finds enough space to store a file and puts it there.
Don't forget you can delete files on a hard disk leaving spaces, the spaces left may not be big enough for some files so they would be stored at the next avaliable space(avoiding defragmentation were possible).
Hope this helps some.