Firewire doesn't engage main computer processor in transfers. It might be irrelevant if you don't do any heavy processing at the same time. There are two different types of USB DACs Synchronous and Asynchronous. Synchronous DAC timing is created (synchronized) by computer's USB bus and traffic on this bus (or computer's activity) can possibly affect timing (jitter). Jitter converts to noise. It is very crude scheme. I suspect that most of new DACs use asynchronous transfer in which DAC is running with its own stable clock synchronizing with computer (to avoid loosing samples) by storing samples in the buffer and telling computer to increase or decrease number of samples per frame. When buffer is too low it sends message to computer "more samples per frame" until buffer is close to be full, then it sends message "less samples per frame".
In this scheme timing of computer is irrelevant, other than overall electrical noise it is creating. Hard disk timing is irrelevant since it contains data (and not the music), meaning - it has no time base.
In this scheme timing of computer is irrelevant, other than overall electrical noise it is creating. Hard disk timing is irrelevant since it contains data (and not the music), meaning - it has no time base.