My MSI Notebook with AMD Dualcore (x64) and ATI HD Graphics has a black screen / freezes at boot of the BT5 KDE x64 Live CD after I read something with AHCI on the screen.
Same happens in Debug Mode.
BT4 works without any problems.
Please tell me if I should provide you more information my notebook (e.g. cpu, chipset, graphics) to help you.