Hi and welcome to the Gigabyte Forum.
Well you can either reflash your BIOS as you did but using the FB version or you can try nad get the backup BIOS to kick in and reflash the Main BIOS. If you want to try this way then follow the next steps:
You can kick in the backup BIOS by shutting down the PSU from the wall, then hold down your case power button in and then turn on the power supply button, a few seconds later the board will start, shut off the power supply then.
Then you can turn on the power supply again and power up the board normally and DualBIOS will kick in.
You said that you changed modes from IDE to AHCI but you must stick with whichever one you used to setup the system and not swap around afterwards.