What’s happening to my iMac

Yes, it does sound like something is shorting out!

Go back undo your HDD and make sure the cables are not caught under the frame as they can compress shorting out.

Did you follow this guide: iMac Intel 21.5" EMC 2389 Hard Drive Replacement and take note of Step 14. Today we need to use this sensor cable: OWC In-line Digital Thermal Sensor for iMac Late 2009 - Mid 2010. Todays HDD’s or SSD’s don’t offer the needed header to gain access to the internal thermal sensor.

You also have a second issue! You need a SATA II (3.0 Gb/s) drive as your system can only support this I/O speed. Today most drives are now SATA III (6.0 Gb/s) only, so make sure the drive you are going with offers SATA II compatibility many don’t! As an example looking at the Seagate SSHD 3.5” FireCuda we can see it fully supports all of the SATA speeds, where as the WD HD 3.5” WD Blue is a fixed SATA III (6.0 Gb/s) drive. The way they documented the port info is a bit confusing as they combined the I/O data rate and the sustained data rates and then they aim you to the SATA specification on www.sata-io.org so bad! Hiding the fact they don’t work in slower SATA I or II systems properly. So if it doesn’t state it clearly don’t buy it!