I see many hard or SSD drives with internal encryption. They often claim to have 256 bit AES. So I can write the data to such disk and data will be stored encrypted. And I can read that data in decrypted state. But how this encryption prevents my data to be read by someone who'll steal my disk?
If manufacturer offers encryption, it's setting is done via software supplied with your hdd. It'll prompt you for a password or another method of authentication everytime you access your drive. But you have to enable this feature.
Advantage of internal encryption is speed of encryption, because it's likely hw based (depends on your manufacturer and type of drive).