Rendering time is extremely CPU intensive, unless you have a professional video editing graphics card. Cards like the RT.X100 from Matrox off-load a lot of the rendering away from the CPU.
I would say the hard drive has very little to do with overall speed. Remember, an average hard drive transfers at 35MB/s or faster. When editing video, the rates usually average below 5MB/s depending on the type of effects you are applying. So adding another hard drive or setting up a fast RAID array will likely have little or no effect.
In your example, the user may have been really low on free space, forcing the hard drive to operate much slower than usual. It's possible that in those situations, adding a 2nd drive with nothing on it helped out a bit. Double the performance though is hard to believe. Upgrading to a professional card is the only way I know of to get that kind of result.
~cdogg
"Insanity: doing the same thing over and over again and expecting different results." - Albert Einstein
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