Heres the deal: The hard drives that apple has for these iMacs have built-in temperature sensors. You can use a non-apple hard drive but the temperature sensor will not be read and consequently the hard drive fan, inside the imac, will run at full speed. The solution is to buy an apple hard drive that is for your imac, the downside is that they are more expensive. Apple has made it so that only the hard drives that they sell work correctly. As I said though the only issue that comes with using a non-apple hard drive is that your hard drive fan will spin at full speed, which can become annoying and likely will decrease the lifespan of the fan. Hope this helps!

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