You should select IDEfix only if you have IDEfix software installed and want continue to use it. In all other cases choose SCSI. The IDEFix choice just does *not* install the >4GB support of OS 3.9.
Hi Thomas,
Sorry to hijack but are you sure about that, I was just about to install the IdeFix express software on my 16GB setup and I don't want to kill it

Update:
Actually it looks like the Express install discs were
patched with TD64.