Than the height. A lot of smaller laptops, including Macbooks, won't take anything bigger than 0.8" high.

I want my disk storage, like my USB keys, as fast and dumb as possible so that encryption is totally under my control.

Anybody that trusts closed and/or unverifiable encryption schemes (whether of operating systems or of intelligent hardware) has basically abdicated responsibility for privacy. The possibilities for malfunction, backdoors and other mischief when storage media demand passwords are frightening to contemplate. We already have enough trouble convincing ourselves that our operating systems are clean without adding intelligent storage to the mix.