Here's a way of appending the contents of one file onto the end of another. Performance-wise it might be slower than reading the full files as demonstrated of previous answers - I've not tested it on 200+mb files, however, because it is using file streams it means that your application's memory requirement won't suddenly spike at 200+mb.
public bool AppendFile(String file_to_read, String file_to_append)
using (FileStream fs1 = new FileStream(file_to_read, FileMode.Open, FileAccess.Read))
using (FileStream fs2 = new FileStream(file_to_append, FileMode.Append, FileAccess.ReadWrite))
byte buf = new byte;
int size = 0;
while ((size = fs1.Read(buf, 0, 4096)) > 0)
fs2.Write(buf, 0, size);
Just pointing out another option available for you.