Is it advisable to run a defragmenter daily? I heard that frequent defragmentation will damage the hard disk. Is it true? If so, how does it happen?
While this question may bring a lot of opinion based answers to the table, I will do my best here to contibute facts and logic to help you make a decision.
When files are written to a disk, the files are split into chunks of data of a fixed size. These sizes are called clusters. For NTFS, this is typically 4KB "chunks". For more information, see This Microsoft Article
Normally, these are written sequentially where there is a space on a disk that is big enough to hold a file. As files get changed and new bits are written, old bits of data are removed etc - there gets to be a lot of fragmentation. Fragmentation is where a complete file is split onto multiple physical areas of a disk. This is a natural process and is a result of normal usage. For more information, see this webopedia article
Defragmentation is the process of moving these chunks of data around on the disk to make sure that full files are written together (again, see here for more). This typically improves performance as the disk head doesn't need to keep jumping around on the disk to get all the chunks of a file.
I personally wouldn't defrag every day... most of my machines defrag weekly or at least go through fragmentation analysis weekly to see if they need it.. but if the disk is kept clean and defragmented, the amount of fragmented data will always be fairly low.
Now onto the question of longevity of a PC. From my own personal experience, I have a windows 95 PC that is now 8 years old... it is still running the same disk that was in it the day it was bought. It gets a defrag once a week and is showing no signs of dieing. While this is circumstantial, I have never heard of a machine being killed by frequence defrag and cleanup.
There is no question that a defrag is intense on a disk, but so is jumpinjg around constantly to look for file fragments... so it's possibly a case of damned if you do and damned if you don't.
I am yet to see any conclusive evidence that a defrag to a disk causes harm when compared to the increased workload of a fragmented disk.. it's the difference between sprinting for a short time (defragging and moving all your fragments together) vs taking a long jog (constantly looking for file chunks),
There is also an argument to be made that if defragging put too much pressure on disks and killed hardware, disk manufacturers would warn against it (haven't ever heard of this happening), OS vendors would stop including their own defraggers (again, never heard of this) or 3rd parties would stop making and selling these tools (again, not happening). There also wouldn't be a thriving market of open source tools written by people which include a defrag as part of PC optimisation.
While all circumstantial, this would imply that there is probably very little danger of a defrag unless your disk was already in a terrible state and ready to die (in which case, the intense activity might push it over the edge, but it was likely ready to die anyway).
Hopefully my long rant helps somewhat.