There are HDDs and SSDs that do the encryption themself, rather than just being a dumb storage medium. You need to unlock it with the password or key when using it, and the disk will transparently encrypt and decrypt the contents on reads/writes.

One problem is that you have to trust the disk (manufacturer/firmware) to do this right. This is not so easy to verify yourself, and not all drives claiming FDE are equally secure. https://en.wikipedia.org/wiki/Hardware-based_full_disk_encry...

https://vxlabs.com/2012/12/22/ssds-with-usable-built-in-hard...