Creating a Huge Dummy File in a Matter of Seconds in C#

Simply create the file, seek to a suitably large offset, and write a single byte:

FileStream fs = new FileStream(@"c:\tmp\huge_dummy_file", FileMode.CreateNew);
fs.Seek(2048L * 1024 * 1024, SeekOrigin.Begin);
fs.WriteByte(0);
fs.Close();

This will yield a 2GB file with basically unpredictable contents, which should be fine for your purposes.

If you don't care about the contents, then by far the fastest way I know of is this - it is practically instant:

private void CreateDummyFile(string fileName, long length)
{
    using (var fileStream = new FileStream(fileName, FileMode.Create, FileAccess.Write, FileShare.None))
    {
        fileStream.SetLength(length);
    }
}

If you just need a FileStream, you could use FileStream.SetLength. That will get you a stream which is 2 GB long. Then you can write the final byte at an arbitrary position of your choice. But the contents will be undefined.

If you're trying to actually create a file on the disk, yes, you'll need to actually write its contents. And yes, hard disks are going to be slow; something like a 1 GB/min write speed isn't totally ridiculous. Sorry -- that's physics!