Thanks for your feed back.
Here's the thing - I can't even boot my old C drive in AHCI mode, using an external docking station. It will only boot in IDE mode but when I installed Windows 7 onto my old C drive, the BIOS was set to AHCI!
So Windows 7 won't boot in AHCI mode from my new C drive that's been re-imaged, or the old C drive, which means it's not the hard drives or the image.
So the question is - why won't Windows boot the C drive in AHCI mode after being unplugged from one SATA port & plugged into another? It makes no sense.