Learn in the textbookApplying Encryption and Digital Signatures
DefinitionDisk encryption (ディスク暗号化) encrypts everything written to a storage device such as a hard disk or SSD.
ExplanationData is encrypted automatically as it is written and decrypted automatically when the legitimate user works with it, so the user rarely has to do anything. The main aim is to keep anyone from reading the contents of a storage device pulled out of a laptop or external drive that was stolen or left behind. If the encryption key is kept in a tamper-resistant chip inside the machine, removing the storage device alone is not enough to decrypt it.
AnalogyIt is like turning the whole archive room into one big safe. Even if the safe is carried off, anyone who cannot open it cannot read a single document on its shelves.