What part of the computer system is most likely to contain data of evidentiary value?

The hard drive is the component of a computer system that most reliably contains data of evidentiary value. It serves as the primary storage device where the operating system, applications, user files, and various types of data (including documents, images, emails, and potentially deleted files or hidden data) are stored. In forensic investigations, the hard drive can hold critical information that can help establish what actions were taken on the system, the timeline of events, user activities, and potentially incriminating evidence.

The microprocessor or CPU primarily processes data but does not store it; thus, it lacks the capability to retain evidentiary data in the same manner as a hard drive. The USB controller manages data transfer between USB devices and the system but doesn't hold data itself. PCI expansions relate to additional hardware components that enhance system capabilities, which might hold minimal temporary data but are not primary storage locations for evidentiary content. Therefore, among the given options, the hard drive stands out as the most significant repository for evidence in a forensic context.