Survival of FDR data unit
Simply by adequate engineering. Desighn depth is close to 20 000 ft. Exposed was the unit to about 13 000 ft. So a lot less than desighn + saftey margin. The hydraulic environment produces a steady load. So if that piece is watertight at the beginning of the period under water it will be watertight even after 2 years or more.
I remember a post in this thread that showed the exact data for that type of FDR, beside the 6000 m depth resistance I thought to remember an impact resistance up to 750 km/h. Itīs apparently a very massive piece made of steel.