No, it is not harmful to defragment your hard drive often.

Files on a Hard Drive become fragmented over time, requiring the disk to work harder and more slowly to access the various fragments. Defragmentation results in faster disk access and less work-load on the disk, but this requires the disk to work hard for a period of time to correct the mess. Some hold that frequently defragmenting a disk actually over-works the drive, but the alternative, a severely fragmented drive also stresses the drive. A happy medium is reached by using 3rd party defragmentation software with automated scheduling features that allow unattended defragmentation. Once defragmented, regularly scheduled defragmentation is performed very quickly because the degree of fragmentation is lower, stressing the drive even less. Hard Drives can and do experience part wear over time and failures can occur, but defragmenting a drive does not offer greater drive stress than various idle processes which run constantly when you are away from your system.

Apart from purchasing a solid state HDD which has no moving parts to fail, those users who are still concerned could follow the steps below.

  1. Monitor hard disk fragmentation over time
  2. Observe the fragmentation rate based on disk usage from their own personal computing habits
  3. Arbitrarily choose a personal fragmentation threshold, say 10% or 15%
  4. Determine how long it takes to defragment based on that threshold figure
  5. Arbitrarily determine a frequency at which to perform defragmentation
  6. After weeks and months of testing, adjust the threshold and frequency figures until you have reached a sweet-spot that you are personally comfortable with