Copy certain lines from one text file to another
It is OK to use File.ReadLines(...) because it actually is shortcut to StreamReader.ReadLine() (Not to be confused with File.ReadAllLines()).



I wonder why you reopen the same file for each line you want to save. I would do something like this:

  byte[] arrayByte = { 0x00 };

  using (var stream = new FileStream(fileName + "_gugggu", FileMode.Create))
  {
    foreach (string line in File.ReadLines(fileName).Where(l =&gt; l.Contains(":10")))
    {
      ...


This may be the real bottleneck.



I think 

      string partialA = line.Substring(9);
      string partialB = partialA.Remove(partialA.Length - 2);


is the same as:

      string subString = line.Substring(9, line.Length - 11); // 11 = 9 + 2




All  in all it could be changed to :

  byte[] arrayByte = { 0x00 };

  using (var stream = new FileStream(fileName + "_gugggu", FileMode.Create))
  {
    foreach (string line in File.ReadLines(fileName).Where(l =&gt; l.Contains(":10")))
    {
      string subString = line.Substring(9, line.Length - 11);
      arrayByte = ToByteArray(subString);
      stream.Write(arrayByte, 0, arrayByte.Length);
    }
  }