Total channel bandwidth
Yeah, sustained transfer rates of most disks hover between 20-30mbytes/sec. That's reads; writes are often 1/3 of read speed or less. It's not the bus speed that's slowing down HDD access, even over Firewire 400. It's the disk itself. However, add a second disk to a Firewire 400 controller, and you may be able to saturate the bus with disk I/O.
For a DAW, read and write speed are both crucial. The reason to use an external, separate drive for your DAW:
* Your internal drive will be used for your page file, which will end up getting accessed while recording
* Your internal drive is regularly used for softsynth patches, which if they're large will frequently be swapped out
* Your internal drive usually gets fragmented much more quickly than a dedicated audio disk
* When recording to a hard drive, you usually want much larger block sizes than you would on an internal disk. You can dramatically improve your read and write speeds by formatting with really big blocks (try instead of 4K, use 16K or 32K)
* The hard disk used for digital audio usually should not be cached at all by the OS or your recording program in order to avoid delayed writes happening in bursts and causing a dropout. Sustained throughput is what you need; bursts are your nemesis.
If you format your external Firewire disk with larger block sizes than the default 4K, disable O/S caching, and use it only for digital audio, you'll notice performance stays pretty darn good. Meanwhile, if you record to your system disk, eventually your wavs will get fragmented, you're stuck with the default tiny block size so throughput will suck, and simultaneous disk accesses will often cause popping in recordings once you're pumping enough tracks through to be near the capacity of your box.
Speaking from experience: use a dedicated, separate drive for audio. Your life will be simpler, faster, and easier to manage. I've never tried it on a USB-attached drive. Just Firewire, SCSI, and IDE for me. I appreciate this board for letting me know to avoid USB for my DAW drives. I'll stick to Firewire.
You *can* do a DAW off the internal system drive, but you'll be able to pump through fewer tracks before your disk I/O is maxxed out. And approaching that limit, you'll start getting pops and clicks earlier. Of course, it's all a matter of perspective. I routinely work with 30-50 tracks running off my mirrored, dedicated audio drives in my desktop DAW. In that case, it's not my hard disk which is usually the problem, it's that my CPU is saturated (thus my desire to replace my DAW). At some point between 30 and 70 tracks, I start seeing my disk usage meter creeping up.
Of course, that's at 16-bit or 20-bit, 48KHz (limitation of my current DAW). If you're working at 24-bit, 96KHz, you're going to approach that throughput limit much faster.
I think back to my "good old days" of using a 4 or 6-track analog recorder, and life is better now