Do you:

(a) Authenticate the ciphertext of the whole device as a unit? So, any time you update any block on the entire disk, you also need to update the MAC tag? What happens if you do that? Any block corruption occurring anywhere on the device breaks the MAC tag for the entire disk.

(b) Authenticate each individual sector? Where do you store the MAC tags? There's an awful lot of sectors on a disk. Also: what does it mean to have a single corrupted sector in a file consisting of many sectors?

(c) ???

Of course, the answer is that full-disk encryption tools (ie simulated encrypted disks, like Truecrypt) don't authenticate. There's no good place to store the tags and no good units of data to authenticate.

What does it mean that encrypted disk sectors aren't authenticated? Well, it means two things:

(i) Attackers can perform targeted rewrites of data on the disk, typically at a full-sector level; in practice, this means that they can randomize any sector of the disk, and the encryption software can't know that the now-random data isn't real data, nor can the operating system. By aiming these random blocks at trusted metadata or executable binary code, attackers can potentially turn this ability into a much more serious attack.

(ii) Because the encryption software must trust the outcome of decryption, even when attackers tamper with data to make its output random, attackers gain the ability to feed chosen ciphertext into the encryption software. This sounds like an academic concern, but it's the basis for many of cryptosystem attack classes we've learned about in the past decade, most notably the padding oracles.