Website - https://support.apple.com/kb/sp689?local...

Upgrades

You can cost-effectively upgrade a number of components in the iMac Intel 27".

  • Memory: If your iMac is still running with only the stock RAM, upgrading will provide a dramatic performance boost at minimal cost. Upgrading to the maximum of 32 GB (four 8 GB modules) is simple and provides unmatched performance.
  • Hard Drive: A 1 TB hard drive came standard with the iMac Intel 27". You can upgrade the drive to 2 TB to expand your storage.

Identification and Background

You can verify that you have a iMac Intel 27" EMC 2429 by checking the processor information through "About This Mac" in the Apple menu.

Use the iMac identification system to help you identify your machine. iMacs tend to look very similar, and it's important to know which machine you have before ordering any replacement parts.

The iMac Intel 27" has multiple external and internal differences from 24" iMacs. Externally, the machine features edge-to-edge glass and an aluminum rear bezel. Internally, an LED display, more powerful video processing, and faster processors round out the impressive upgrades. Additionally, the 27" iMacs have the build-to-order option of a Quad Core i5 or i7 processors to give you the most powerful iMac experience yet!

The iMac Intel 27" EMC 2429 Is largely unchanged from its predecessor with the exception of some internal lay out and an inconvenient change to the hard drive; the internalizing of the thermal sensor. Moving the thermal sensor into the hard drive has done one major thing for this computer, it has made changing hard drive changes expensive and options limited. The thermal sensor wiring is now part of the SATA cable and is handled by proprietary firmware in the hard drive. In other words, even if you manage to attach an after market drive, you will be stuck with fans running at full blast unless you can find a fan controller.