It's funny because "hard drive" no longer means exactly what it used to mean.  Obviously the original meaning was about a specific  technology (involving multiple spinning magnetic media platters and magnetic read/write heads).  Now we have usb devices that hold huge amounts of data (several gigs) which are marketed as thumb drives or hard drives.  So the name has come to mean the function rather than the actual specific type of manufactured device.  The canonical definition is non-volatile permanent storage.  Access times in traditional hard drives could be improved by spinning the platters faster.  Contrasting these attributes with the transient property of memory can aid understanding.