I am used to having to Defragment hard drives on the PC. Is this necessary on the Mac and if so is there a good program to buy to do it or does this come with the computer?
No. Defragmenting is a Windows solution to a Windows problem. To the extent that it is necessary on the Mac, the OS automatically defrags files. Apple recommends that you do not use a defrag utility unless you are one of the very rare birds who edits large video files.
Defragmentation on HFS+ volumes should not be necessary at all, or worthwhile, in most cases, because the system seems to do a very good job of avoiding/countering fragmentation.
It is risky to defragment anyway: What if there's a power glitch? What if the system crashes? What if the defragmenting tool has a bug? What if you inadvertently reboot? In some cases, you could make the situation worse by defragmenting.
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Conclusion
Defragmentation on HFS+ volumes should not be necessary at all, or worthwhile, in most cases, because the system seems to do a very good job of avoiding/countering fragmentation.
It is risky to defragment anyway: What if there's a power glitch? What if the system crashes? What if the defragmenting tool has a bug? What if you inadvertently reboot? In some cases, you could make the situation worse by defragmenting.