Hardware issue on iMac, flashing question mark I have an iMac 2008/2009 issue and recently the computer would not boot and I would get a flashing question mark. Ran through all the usual steps, try boot in safe mode, flash RAM,  try single user for fdisk, put in OS disk and boot from CD, etc... No dice. 

Replaced the hard drive and put the OS disc in the system, press and hold option. (holding C didn't work) I can see the OS disk but when I click on it the system will act like it is going to run and then freeze. The best I got was the OS X disc that changed the screen to dark grey and told be the system had to restart.

Purchased a usb mount for an external hard drive. Was able to mount the old hard drive to a laptop and run disk utility. Everything checked out.

At this point I am pretty sure it isn't the hard drive. 

Any other hard ware items this can point too? My iMAC can also "see" the hard drive (and the recovery partition) when connected via the USB and I hold "option" on start up. But it crashes when trying to boot from either.

I am going to take out and re-install the RAM for giggles but I am not sure what else to do. 

Ideas are welcome.
If you see a folder with a question mark appear instead of the Apple logo, it means your Mac couldn't find a local or network-based startup disk. This can happen if the disk selected in the Startup Disk pane of System Preferences isn't available. Wait a few seconds to see if your Mac is able to locate the startup disk you specified. 

If you still see a question mark after waiting a few seconds, use Startup Manager to start up your Mac, then select your startup disk from System Preferences.

If a question mark appears after you install a software update, select your startup disk again using macOS Recovery (Power off Mac, then power it on while holding Command+R until you see the Apple logo).

Reference: https://support.apple.com/en-us/HT204156