You should check how much was read before write, something like this (and you don't need to remember the offset to seek, the seek is automatic when you write):

int read;
do
{
    read = responseStream.Read(bytesInStream, 0, (int)bytesInStream.Length);
    if (read > 0)
        fileStream.Write(bytesInStream, 0, read);
}
while(read > 0);