string myData  = File.ReadAllText("hexData.txt");
string newData;
int remainder  = myData.Length%500;
byte[] data_toWrite=newByte[250];

for(int i=0;i<myData.Length-remainder; i+=500)
{
    newData     = myData.Substring(i,500);
    data_toWrite = StringToByteArray(newData);
    File.WriteAllBytes("video.mjpeg",data_toWrite);
}

newData     = myData.Substring(myData.Length-remainder,remainder);
data_toWrite = StringToByteArray(newData);
File.WriteAllBytes("video.mjpeg",data_toWrite);

public static byte[] StringToByteArray(String hex)
{
  int NumberChars = hex.Length;
  byte[] bytes = new byte[NumberChars / 2];
  for (int i = 0; i < NumberChars; i += 2)
  bytes[i / 2] = Convert.ToByte(hex.Substring(i, 2), 16);
  return bytes;
}