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I want to replace my hard drive with a much larger one. My system info says I have a Hitachi but I don’t see that under available replacements. I need specific replacement for my machine because of the thermal sensor I believe. Any guidance?

Luckily your model was just before the proprietary heat sensor issue. The heat sensor simple sticks onto the hard drive externally. So you can pick any brand of 3.5" hard drive you want. Just make sure it is backward compatible to SATA 1, 1.5 GHz. Here’s the repair guide: iMac Intel 20" EMC 2133 and 2210 Hard Drive Replacement