Can I replace the western digital hard drive with a seagate hybrid

Well the answer is Yes & No!

The drive is not an issue, its just a standard SATA II (3.0 Gb/s) drive. The issue is the system leverages a thermal sensor within the system to monitor the temp of the drive to then alter the systems cooling via SMC.

If you take a look at iMac Intel 21.5" EMC 2389 Hard Drive Replacement Step14 you’ll see the connection of the thermal sensor. Now the issue is twofold, each drive maker sets up a different set of pins in the header for the diagnostic sensor Apple is leveraging so you need the correct cable per the given drive.

Now the second issue is the current drives don’t offer the needed interface header to even attach to the internal sensor (some don’t even have a sensor at all). So we need to install an alternative thermal sensor so the system runs properly. As it turns out OWC offers the needed part OWC In-line Digital Thermal Sensor for iMac Late 2009 - Mid 2010 Hard Drive Upgrade. With it you can install any SATA II (3.0 Gb/s) HDD or SSD drive.

Which brings me to the next issue getting a compatible drive. Make sure you review the given drives spec sheet make sure it lists SATA II (3.0 Gb/s) if not move on to a different drive. Finding a 3.5” drive that supports the older SATA II spec is getting harder. Some drives are auto sense like the Seagate FireCuda SSHD Note the line ‘‘SATA Transfer Rates Supported (Gb/s)’’ which lists 3.0 (SATA II) its one of the few that does support SATA II systems.