Before starting a bare-metal installation of Windows 10, ensuring that there is sufficient storage space on the system's hard drive or SSD is crucial. This is because the installation files need enough room to be copied and for the operating system to be installed and function properly. Without adequate storage, the installation process can fail or the operating system might not perform optimally. Other prerequisites like internet connection, product key, and specific firmware types might be necessary at different stages of installation or activation, but the fundamental requirement is enough storage space to accommodate the new OS.