You can use methods like File.AppendAllLines() in C# to add new data to an existing text file.

However, the search results do provide information on the legal, technical process of creating text data dumps in various programming scenarios. If you are looking to write data to a .txt file, here are the methods described: Technical Methods for Writing Data to .txt Files

You can log data from serial ports into a .txt or .csv file using software like CoolTerm.

If you are working on a data logging project, I can provide: for writing data to files. Arduino code for logging sensor data to an SD card. Let me know what kind of data you are trying to save! Function to dump SD text files over serial - Arduino Forum