A sure sign that your computer is thrashing is when an application stops responding but the disk drive light keeps blinking on and off. Thrashing is generally caused by too many processes competing for scarce memory resources. To temporarily stop thrashing, you need to terminate one or more applications. To stop it permanently, you need to install more main memory.
Disk thrashing can result in permanent failure of the hard drive; as the data is transferred back and forth, the hard drive s read/write heads are subjected to considerable wear and tear.