Let's be pendantically accurate here.
A mechanical hard disk (i.e. not solid state), like any mechanical device, is prone to wear and tear as it is used. A heavily used hard disk will wear out faster than a lighter used hard disk.
Defragmenting a disk does involve a fairly heavy use of the disk so defragmenting a disk will cause a slightly higher wear and tear on the disk mechanics than normal use. However, unless you are running a particularly aggressive defrag continuously 24/7, the reduction in the expected life time of the disk is miniscule.
On the other hand, accessing a file on a heavily fragmented disk is going to involve more head movement than accessing the same file on a defragmented disk. So normal use of a heavily fragmented disk will cause more wear and tear than the same use on a defragmented disk. Again the extra wear and teat will be miniscule.
So, yes, defragmentation will cause some additional wear and tear on the disks but (a) that wear and tear will be miniscule and (b) any wear and tear will be offset by the reduction in wear and tear in normal use.
The situation is different on solid state disks - whereas in a mechanical disk there is some wear and tear for both reading and writing data, with a solid state disk there is practically no wear and tear for reading data regardless of how badly fragmented the disk is (and there is no performance impact of fragmentation on solid state disks). However, there is significant wear and tear when writing data (significant compared to a mechanical disk - a solid state disk will still last at least 5 years under fairly heavy use). So defragmentation is not recommended for SSDs.
Matthew
