I replaced my mechanical HDD with a Solid State months ago, no issues.

I came to sell the iMac and stupidly forgot about the weak internal DP connector, ripped it off.

So i bought a replacement board: 661-5429 Logic Board - 27 inch 2.66 GHz i5 iMac Late 2009 A1312 | eBay

I installed the new board and tried to boot, apple logo appeared and hard power off and reboot.

I held Option, and tried to boot of MacOS Install DVD, apple logo appeared and hard power off and reboot.

So I thought, this is bad news.

Then

I held Option, and booted of my Bootcamp partition, perfect, burn in tests etc run fine in windows.

Have I missed something or is it a dodgy logic board?