system() copy fails, while cmd copy works

#include <iostream>

using namespace std;

int main()
{
    system("copy c:hello.txt c:hello2.txt");
    system("pause");

    return 0;
}

system("copy c:\hello.txt c:\hello2.txt");

FILE *fh = fopen ("c:text.dat", "w");

system("copy c:/hello.txt c:/hello2.txt");