How can an adversary request decryption of an unused sector? The Wikipedia article on Disk Encryption Theory mentions this


  a powerful adversary can modify an unused sector on the disk and then request its decryption.


I understand that an adversary is capable of modifying an unused sector, but can anyone explain how they would be able to request its decryption?
I understand that an adversary is capable of modifying an unused sector, but can anyone explain how they would be able to request its decryption?


The first thing to understand here is that this is an adversary model. In cryptography we model our adversaries so we know exactly what capabilities they have and which they don't.
In this particular case it seems whoever came up with this model decided that it would be a good idea to give the adversary the ability to change the ciphertext of and decrypt unused sectors.  

Now we could model our adversary so that they don't have this capability. But in general we want to come up with models that are as powerful as possible so that if we prove a scheme secure in that model, it provides the maximum protection possible.
The only limit to the powers of adversaries that we usually use is we don't give them powers that always break any scheme we could possible come up with. E.g. when we encrypt a challenge bit for the adversary we do not allow the adversary to ask for the decryption of that specific ciphertext. 

So the default in cryptography is to always add powers right until we barely don't get trivial breaks of security. Now for this specific capability. It seems to be useful because it utterly breaks the ability of the disk encryption to use an unauthenticated deterministic stream cipher (e.g. initialized / keyed by the sector index). This is because an adversary can just look at the ciphertext, request a decryption and then compute any such static keystream as an XOR. The keystream could then later be used if the sector becomes used at some point to extract data and break security. Additionally if the scheme allows you to get some value out of knowing plaintext-ciphertext pairs (e.g. block-indexing parameters), getting them from an unused sector may prove useful, especially if the decryption capabilities of used sectors are somehow limited.