also what do you mean by scratch disk.
I don't mean "scratch disk" in the Photoshop sense (basically a swap file.)
A dedicated drive for importing/editing/exporting improves performance, takes load off your boot drive, and decreases the likelihood of random I/O from the operating system and/or housekeeping programs interfering with video captures.
The faster the drive, the more simultaneous streams of data can be read and/or written. (MOAR is BETTAR!)
Once upon a time, I did it all on a single drive. (Hey, I was young.) An automated virus scan started while I was capturing. 90 minutes later, when I came back, I had to start over.
Life is too short not to use a dedicated drive.