Host Protected Areas
Note that many drives will contain a host protected area, which will not be displayed in the file explorer for an operating system. The MAX ADDRESS of a drive can be set so less than the full size of a drive is shown. The rest of the drive will be a host protected area, the OS cannot work with. But BIOS software may work with the host protected area. A host protected area may be required for some firmware because it cannot successfully boot from a large drive.
Host protected areas can also be used to hold system restore software.
The host protected area will not be altered when a drive is reformatted, so theft detection software may be loaded there.
Naturally, the host protected area is also a good place to hide data, including malware.