Over the past twenty years, Secure Digital memory cards have become the most popular digital storage media. Therefore, law enforcement agencies need to develop forensic techniques to enable recovery of data, especially from cards severely damaged by accidents, air crashes, terrorist attacks or deliberate attempts by criminals to destroy evidence. The paper discusses the non-invasive and invasive diagnostics available to legal forensic experts, with descriptions of the necessary equipment, including binocular microscopes, X-Ray equipment, scanning acoustic microscopes, chemical benches, and, for the first time, infrared cameras. All of these techniques can be used to methodically determine the best treatment in order to repair damaged storage media and extract data from them. The main contributions of the paper include the development of an innovative systematic forensic protocol for diagnostics on damaged cards, which involves a simple decision-making diagram. Finally, a concrete case study is presented using the new forensic decision diagram-based protocol and the panel of techniques available to diagnose a card damage.