Ok I unplugged my hdd and cold booted three times in a row, no problem.
Then I plugged it back in and cold booted, no problem.
Tried it again, worked fine. Tried it a third time and it didn't post. Unplugged the hdd again to make sure it definately was the drive and guess what, this time it didn't post. After 10 mins of plugging and unplugging different hdd's it seems there is no pattern to this behaviour. Sometimes it posts fine with or without any hdd's plugged in, sometimes it doesn't. Sometimes it posts while the system is still on after making a change to the bios, sometimes it doesn't. Seems to me this issue isn't related to my hdds. What next???