Here's an excellent article on upgrading your 27" 2009. It also has links to compatible drives: http://www.everymac.com/systems/apple/imac/imac-aluminum-faq/imac-intel-aluminum-how-to-upgrade-hard-drive.html
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UPDATE
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The problem with changing drives is the thermal sensor. If you stay with the same "brand" of drive there should be no problem. If you change brands, you will need to change to that companies sensor. As far as I know there are three different ones, the Western Digital 922-9225, the Seagate 922-9224 and the Hitachi 922-9223
Here's an excellent article on upgrading your 27" 2009. It also has links to compatible drives: http://www.everymac.com/systems/apple/imac/imac-aluminum-faq/imac-intel-aluminum-how-to-upgrade-hard-drive.html