Mac OS X parks the 'heads' of your HD as much as it can to protect the hard drive should the laptop suddenly move, and to conserve power. Unfortunately, it happens so much that it causes 2 problems:
1) Each tick is known as a 'load cycle', and too many load cycles causes premature HD failure
2) On some drives, the tick is loud enough to be highly annoying in a quiet room. But, Apple's hitachi drive didn't seem to be loud enough that I noticed it at all. I only noticed it when I put the WD drive in.
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Now, this stops the drive from EVER parking the heads, so you will never hear the ticking again
BUT, it causes two more problems:
1) The head parking does actually protect the drive from movement damage, so you are increasing the chance of failure
2) It messes with the automatic sleep function which means you have to close the laptop to get it to sleep, or go to apple > sleep. As long as the laptop is left open, the hard drive will keep spinning and spinning forever!
AND, you have to open terminal and type 'hdapm disk0 max' every time you reboot and sometimes when you switch to battery power.
Taken from macrumors article
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