Final answer:
To use BitLocker encryption without an external startup key, enable the TPM in the BIOS, allowing the system to boot automatically while keeping the data secure.
Step-by-step explanation:
To implement BitLocker encryption on a hard disk and allow the system to boot automatically without a startup key on an external USB device, you should enable the TPM (Trusted Platform Module) in the BIOS. This option is designed to work with the TPM, which stores the encryption keys securely and releases them only if the system's boot components are unchanged. Using a TPM alone allows for automatic booting of the system while still maintaining encryption without needing a PIN or a startup key on an external device.