There are two types of USB devices on which private keys and public key certificates can be stored, a USB cryptographic device and a USB flash memory device. Each device can support a different key and digital certificate storage formats.
The correct and most secure method is to store the private key and public key certificates on a USB cryptographic device (also commonly referred to as a security token, hardware token or a cryptographic token). A Digi-Token™ is a cryptographic token.