In a typical DAW, you want a "boot" HD (OS/Applications)... and a HD dedicated to streaming audio files.
If you'll be using disk-streaming sample libraries (ie: Kontakt 3), you should also consider using a 3rd HD dedicated to streaming samples.
Regarding speed of external HDs:
eSATA - best choice for maximum performance yields full speed of the HD - ~104-107MB/Sec with a fast HD
1394b (Firewire 800) - yields ~70+MB/Sec
1394a (Firewire 400) - yields ~37MB/Sec
USB 2.0 - yields about the same speed as 1394a, but with higher CPU use
For backup purposes, any of the above is fine.
For absolute maximum performance, eSATA is the way to go.
post edited by Jim Roseberry - 2008/12/22 08:40:59