The postman doesn't deliver many mails, and neither does the sendmail program. A cracker, on the other hand, is someone engaged in criminal activity. They might also be a hacker, but do not conflate these two terms. It is an awkward and unnatural invention. Floppies are diskettes.
CDs are disks.
Hard drives are neither, any more than tape drives are tapes, or wheels are tires. Hard drives can, however, contain disks. Sometimes they don't, though. Do not confuse the container with what it contains. I have network file systems, RAM file systems, floppies with file systems, removable disks with file systems, and regular disks with file systems.