"This week Apple released an update for Intel-based systems running Mac OS X versions 10.5 (Leopard) and 10.6 (Snow Leopard). Performance Update 1.0 is available direct from Apple for Snow Leopard and Leopard. It can also be downloaded via Software Update."
Basically this fixes the hard drive pausing issue that some are experiencing. Not sure your Mac is affected? You can go to the support page from Apple and see if it is on the list. If you need it, you can get it via Software Update on your Mac or go the Leopard or Snow Leopard update pages and download manually. This does require Mac OS X 10.6.1 or Mac OS X 10.5.8.
Enhance Your Performance With This Update
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