Memory loss fail-safe system

From Memory Alpha, the free Star Trek reference

The memory loss fail-safe system was a defense built into the programming of Lieutenant Commander Data. In the event of a memory loss, it was intended to protect the android from anyone who might try to take advantage of him.

Data's ethical and moral subroutines would take over all his basic functions. Effectively, he entered a self-preservation mode in which he only knew right and wrong.

The system was activated in 2375 when some of Data's memory engrams, located within his positronic matrix, (Star Trek Nemesis) were damaged by a Son'a weapon on Ba'ku. (Star Trek: Insurrection)