With my computer I have my main HD partitioned between a C drive for OS stuff and a B drive for all my games/media/etc.

I know during install I was forced to install to the C drive and had previously contact tech support about it a few weeks ago. After the install had hit 100% I was able to just cut and paste the entire Diablo 3 folder over into the B Drive though with no issues. Just thought I'd share in case anyone else was curious about this.