A hard drive crash and hard disk crash are terms used to generally describe the failure of computers even though the failure could be due to the many other components that are responsible for the computer functioning. Examples of these components are the CPU (central processing unit), the mother board, the power supply, the IDE (integrated drive electronics) controller, software, etc.
More specifically, a computer crash and disk crash refer to the failure of the hard drive within the computer. Since the hard drive stores all the software for the computer, a failure in this element causes the computer to not function properly or at all.
The actual hard drive or hard disk may fail for several reasons and the term hard drive crash refers to one of those specific reasons. A hard drive consists of rigid disk platters that are coated on both sides with magnetic material. One or several of these disks are found within a hard drive case along with mechanical parts to make the drive function such as a motorized hub to spin the disks and very small magnetic heads to read the hard disks. There is one magnetic head attached to a moveable arm for each side of every disk within the hard drive case. Thus, a hard drive that contains two hard disk platters has 4 moveable arms with 4 read heads (one for each arm).
The disks within a hard drive spin at very high speeds. The speeds vary depending on the hard drive and size of the disks, but a common speed is 7200 rpm. This high spinning speed causes the read heads to float over the hard disk when the disk is being used. The air cushion between the surface of the hard disk and the magnetic head is very small and there is no physical contact between the magnetic head and the hard disk when it is in use. If the heads do contact the surface of the hard disk when the disk is spinning then this is called a hard drive crash and because the surface of a hard disk is delicate, irreversible damage such as scratching can occur. This will lead to loss of information and likely a hard drive that no longer functions.
A hard drive or computer crash can occur because the computer is moved while in use, because the surface of the hard disk has been contaminated, or simply because of malfunctioning parts within the hard drive.