How to write a large buffer into a binary file in C++, fast?

I’m trying to write huge amounts of data onto my SSD(solid state drive). And by huge amounts I mean 80GB.

I browsed the web for solutions, but the best I came up with was this:

#include <fstream>
const unsigned long long size = 64ULL*1024ULL*1024ULL;
unsigned long long a[size];
int main()
{
    std::fstream myfile;
    myfile = std::fstream("file.binary", std::ios::out | std::ios::binary);
    //Here would be some error handling
    for(int i = 0; i < 32; ++i){
        //Some calculations to fill a[]
        myfile.write((char*)&a,size*sizeof(unsigned long long));
    }
    myfile.close();
}

Compiled with Visual Studio 2010 and full optimizations and run under Windows7 this program maxes out around 20MB/s. What really bothers me is that Windows can copy files from an other SSD to this SSD at somewhere between 150MB/s and 200MB/s. So at least 7 times faster. That’s why I think I should be able to go faster.

Any ideas how I can speed up my writing?