What is the best preventive measure for a company that recently had sensitive data stolen via a flash drive?

The best preventive measure in this scenario is to restrict the usage of flash drives in company policy. This action addresses the root cause of the data theft by controlling how sensitive information can be accessed and transferred. Given that the data was stolen via a flash drive, creating a policy that limits or prohibits the use of such devices can significantly reduce the risks associated with portable storage.

By implementing such a policy, the company is taking proactive steps to protect sensitive information from unauthorized access and transfer. This approach can also include additional training for employees on the risks associated with using external storage devices and alternative methods for transferring data securely within the organization.

The other measures, while they can enhance security, do not directly tackle the issue of unauthorized data transfer through flash drives. Disabling unused physical ports, implementing BIOS passwords, and installing chassis locks contribute to overall server security but do not specifically prevent the unauthorized use of flash drives, which is the immediate concern related to the theft incident.