for (int i = 0; i < countOfParts; i++)
{
    thread = new Thread(CreateFileParts);
    thread.Start(i);
}

 void CreateFileParts(object i)
        {
            try
            {
                lock (locked)
                {
                    using (FileStream fsRead = new FileStream(fileName, FileMode.Open, FileAccess.Read))
                    {
                        //for (int i = 0; i < countOfParts; i++)
                        {
                            using (FileStream fsWrite = new FileStream(@"parts" + $"{i}_{fileName}", FileMode.Create, FileAccess.Write))
                            {
                                if ((int)i == countOfParts - 1)
                                {
                                    byte[] buffer = new byte[sizeLastPart];
                                    fsRead.Read(buffer, 0, buffer.Length);
                                    fsWrite.Write(buffer, 0, buffer.Length);
                                }
                                else
                                {
                                    byte[] buffer = new byte[sizeOfOnePart];
                                    fsRead.Read(buffer, 0, buffer.Length);
                                    fsWrite.Write(buffer, 0, buffer.Length);
                                }
                            }
                        }
                    }
                }
            }
            catch (Exception)
            {
                Console.WriteLine("error");
            }
        }

using (FileStream fsRead = new FileStream(fileName, FileMode.Open, FileAccess.Read))
{
   for (int i = 0; i < countOfParts; i++)
   {
      using (FileStream fsWrite = new FileStream(@"parts" + $"{i}_{fileName}", FileMode.Create, FileAccess.Write))
      {
         if ((int)i == countOfParts - 1)
         {
            byte[] buffer = new byte[sizeLastPart];
            fsRead.Read(buffer, 0, buffer.Length);
            fsWrite.Write(buffer, 0, buffer.Length);
         }
         else
         {
            byte[] buffer = new byte[sizeOfOnePart];
            fsRead.Read(buffer, 0, buffer.Length);
            fsWrite.Write(buffer, 0, buffer.Length);
         }
      }
   }
}