Breaking encrypted file store with a file? Let's say you have an encrypted file store of some kind such as an encrypted hard drive, ZIP file, etc.  You don't have the private key or password to this file store, so a brute-force attack is not a good option.

But what you DO have is a copy of a file -- a file that you know for certain is on that file store.

Does this help reduce the complexity of an attack? In other words, would I be able to crack the encryption in less time by knowing part of the contents of the encrypted file store?

(Forgive me if this is obvious, encryption is not my specialty.)
This is called the known-plaintext attack. Any cipher algorithm which is prone to this type of attack is considered very weak. Therefore, AFAIK, no present-day cipher (e.g. AES) has this weakness. Even DES needs 243 known plaintexts to be broken under linear cryptanalysis. (see this topic  on Wikipedia).

On the other hand, all hope is not lost. Weak ciphers or bad implementation of strong ciphers are still commonplace. One noticeable case is with the (old) implementations of ZIP. More info is available here:

http://www.elcomsoft.com/help/archpr/index.html?known_plaintext_attack_(zip).html