DAW best practice is to have your OS(and its supporting swap file) on a separate drive from the one used to receive and playback the audio data stream.
As to the question of internal vs. external from a performance standpoint, the location of the drive is not the determining factor. You could take your internal ATA or SATA device and move it outside the computer's case(stringing power and interface cables from inside). Assuming no extreme environmental factors were encountered that would degrade drive performance and that cabling length remained within spec, the data transfer rate would remain the same.
The real issues that impact drive performance are the drive itself(RPM, read, write, seek, and buffer performance) AND the ability of the computer's interface to shove the data from one place to another. This is an area in which a huge amount of misinformation exists.
For purposes of any data transfer, whether to a hard drive, over a network or you name it, the effective throughput is only going to be as fast as the weakest part of the data chain. When USB devices first started to enjoy widespread use, many people were claiming that their USB audio interfaces were outperforming PCI-based(internal) interfaces. Let's look at the math on that question since part of it applies to hard drives as well:
Bus Width Clock Throughput
USB2.0 1 bit 480mHz 60MB/s
PCI 32bits 33mHz 133MB/s
Remember, we are needing to move 8 bits of data to write a Byte on the disk.
Using this formula-
Data throughput in BYTES/sec = (bus widthXclock rate)/8
For USB2.0:
(1 X 480,000,000)/8 = 60,000,000B/s
For PCI:
(32X33,333,333)/8 = 133,333,333B/s
Many people look at the clock rates of USB and Firewire and assume that this means that more data is being moved. In fact, since both USB and Firewire are both serial standards, meaning that data is moved down the bus one bit at a time, 33mHz PCI (by moving 32 bits of data on one clock cycle) can easily outstrip both USB 2.0 and Firewire 800, which are still the most commonly sold external hard drive interfaces. Note that the current most widely sold internal hard drive interface today is SATA which is even faster than 33mHz PCI. Also, note that PCI has given way to even faster internal bus specifications like PCI Express x16 which can move a whopping 4000 MB/sec, but is still in widespread use.
Here are the figures for the most commonly sold internal hard drive types in the last 5 years:
Device type Bandwidth
ATA 100 100MB/s
ATA 133 133MB/s
SATA 150(1200Mbps) 150MB/s
SATA 300(2400Mbps) 300MB/s
Note that the slowest of these, ATA100 still exceeds the bandwidth of both USB2.0 and Firewire(IEEE1394b), 60 and 98 MB/s respectively.
Once the bus has reached the level of SATA 150, the fastest hard drives in widespread consumer use today can barely saturate the bus(due to physical limitations of the drive itself), so that SATA 300 can only exhibit an effective performance bump when talking to an array of devices or to non-mechanical media(read RAM). The single spinning disk media is (as of yet) unable to take advantage of that level of throughput.
Still, in terms of hardware most commonly found in a current day DAW, the internal vs. external debate is most likely a SATA 150 internal drive vs. an external device on either USB2.0 or Firewire 800(IEEE 1394b). Comparing the bus performance alone, here's the score:
Externals
USB2.0 60MB/s
Firewire 800 98.3 MB/s
Internal
SATA150 150 MB/s
...or, to quote PC World magazine in a recent article :
"External hard drives aren't as fast as internal models, but they are great for backups and are easy to install."
The advent of eSATA has the potential to level the throughput playing field. It will be interesting to see how large a splash it makes in the market.
