You OS and apps need to do a lot of housekeeping work.
Anything that pulls the read/write arm off of what it's doing and to another part of the disk interrupts the flow of data.
(even with ssd's your drive can only do one thing at a time)
You don't want ANYTHING mucking with the smooth flow of data while recording/mixing.
If the interruption of data overflows the data buffers you'll get clicks, pops or total dropouts in the sound.
A smooth uninterrupted flow of data is your goal.
I've never seen my computer doing more than about one block read or write per second beyond what a foreground app is explicitly telling it to do. If your DAW is so sensitive to latency that such a negligible amount of activity is going to cause a problem, you're way beyond screwed.
If your computer is significantly more active than that while idle, it usually means that your computer is doing background paging while idle, in which case you have no prayer of being able to do audio recording even if you add an external drive. Add RAM instead.
Bear in mind that data flow from the hard drive is anything but smooth. A typical DAW reads several megabytes at a time from each track, because that results in the best I/O throughput. So it might, for example, read ten, twenty, even thirty seconds of audio from each track that you're simultaneously playing. If the DAW doesn't suck, it then requests the next chunk of tens of seconds well in advance of when it needs it.
And VIs typically keep all of the current sample set in RAM. There's no way that they could realistically load sample data from disk quickly enough to react to a MIDI key down event. Bear in mind that unlike audio tracks (where the DAW can pre-load the data ahead of time), a VI learns what note it is going to play approximately when it is time to play that note. If you aren't playing the VI live, it gets the command a few milliseconds ahead (as requested by the VI) to allow it time to handle the effects processing and buffering, but VI latency has to be an order of magnitude smaller than hard drive latency (or more). The data has to be there in memory and ready to play, not coming back from the disk tens of milliseconds later.
Hard drives cannot react to requests in anything approaching real time. The minimum average round-trip latency for a 7200 RPM drive is somewhere around 5 milliseconds (and realistically, the average latency is longer than that). That's an eternity. If you tried to play five notes at once that required the VI to load five different samples, you'd have to wait a whopping 25-50 milliseconds (an audible delay) if your VI loaded the sample data from disk on demand. That's just not realistic. Hence, your VIs prefetch all of the samples that they are likely to need during a given playback session and store them in RAM. They should not be hitting the disk
at all during playback.
So unless your equipment is ancient or your DAW sucks, glitches in playback are
never caused by disk performance problems. A modern HD will handle hundreds of simultaneous tracks at typical sample rates without a hiccup. Glitches in playback are typically caused by either interrupt latency (caused by poorly written device drivers) or by a poorly written DAW that doesn't pre-fill its buffers far enough ahead of time.
The best setup for ANY daw is:
a) OS, apps and plugins on boot drive
b) Sample libraries on a separate drive
c) audio tracks and projects on a third drive
The best setup involves one or more SATA-attached drive for everything. An external USB drive will almost invariably hurt more than it helps because USB's high latency more than exceeds the latency improvement you get from having the data on a separate drive. An eSATA drive would theoretically help in the unlikely event that you are working with a high enough track count for disk performance to matter. Again, though, unless you're approaching 90 tracks (@24/96) on a modern laptop drive or 150 tracks (@24/96) on a modern desktop drive, you are almost certainly
not I/O bound.
Oh, and to answer the original question, I use an external drive for backup purposes. That's what external drives do well. If I absolutely can't squeeze enough storage into a particular box for what I'm doing (e.g. when doing lots of multichannel video work), then I add additional external eSATA drives, but I do that only if I can't buy a big enough internal drive to hold everything. Given a choice, additional internal drives almost invariably provide better performance with fewer problems, assuming comparable SATA controllers (unless your computer uses a laptop drive internally, in which case an eSATA drive will be faster, if only because it would typically be a desktop drive instead of a laptop drive).
