Tried Installing new HDD in Alum. iMac. Won't Boot :(

[fluffyx]

Remove the glass panel, and remove all the screws that attach the aluminum bezel. With the screws removed, power on the computer and see if your problem is solved. If it works properly with the screws removed, one of the longer screws was inserted in a place where one of the shorter screws should go.

If your issue is not fixed with the screws removed, it's possible there's a damaged or loose cable, another grounding issue (long screw placed where a short one should be), or electrostatic discharge damage to the main logic board.

 

[GimmeSlack12]

Remove the glass panel, and remove all the screws that attach the aluminum bezel. With the screws removed, power on the computer and see if your problem is solved. If it works properly with the screws removed, one of the longer screws was inserted in a place where one of the shorter screws should go.

If your issue is not fixed with the screws removed, it's possible there's a damaged or loose cable, another grounding issue (long screw placed where a short one should be), or electrostatic discharge damage to the main logic board.

Little late to the party Fluffy.
Anyway, got the iMac back and installed the 1.5Tb drive. It's ALIVE!

Ultimately my problem here was that I did not plug the LCD power cable back in tight enough and tried booting up the iMac. This burned out the Power Supply Unit (PSU). Lesson learned. Case Closed.