A USB Drive is the only device or component that can be used as a boot device. DDR2 is a type of Random Access Memory (RAM). HDMI (High-Definition Multimedia Interface) is audio/video interface. FDISK is a command on some operating systems that provides disk partitioning functions - but it is not a storage device as requested by the question.
USB drive may refer to:
A USB flash drive or "thumb drive", a USB-connected computer storage using semiconductor non-volatile random-access memory
A USB external drive, a hard drive fitted with a USB interface
Secure Digital, a non-volatile memory card format
CompactFlash, a flash memory mass storage device
Memory Stick, a removable flash memory card format