firewire or USB for external music storage drive?

No, not really. Firewire 400 is generally a little faster than USB2, despite USB2's faster theoretical maximum speed (480 mbps vs. 400), but both are far faster than the rates needed to support audio. Firewire 400 is preferred for video editing work, because the protocol guarantees that the transfer rate will never drop below what is needed to handle the dv (digital video) format without dropouts, while USB's transfer rate will fluctuate and occasionally drop below that minimum. But for audio either should be fine.

Regards,
-- Al

Firewire is probably the better choice if you are using a USB DAC. Every USB item you attach will share the bus with any others that happen to be demanding throughput at the time. So if you're running a USB DAC and a USB hard drive they will split the USB bus (and divide it further should you use any other USB devices at the same time). Using a Firewire drive will give that much more dedicated throughput to your USB DAC (or in your case your HAG converter). As Almarg implies, audio is not that demanding in terms of transfer speeds, so all of this may be a non-issue, especially if your computer is doing nothing but serving your audio signal.

USB will rob the processor of cycles, while the firewire will not.

all good except that you will find many more USB DACs then FW DACs...