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Would it be safe to upgrade to the 7200 RPM drive in this iMac or would overheating be an issue? And would it require a temp sensor to make it work?

Here’s what you need to install any third party drive. https://eshop.macsales.com/item/OWC/DIDI… For better performance go with a SSHD. https://www.newegg.com/Product/Product.a… Without the OWC sensor the fans will run at full blast.

An upgrade to a 7200 RPM drive would be no-problem, Just be sure to purchase a fan control software because apple’s Hard Drives have special temperature probes.**iMac Intel 21.5" EMC 2428 Hard Drive Replacement