Let's say I have three 1TB drives - A, B, and C. A contains 1TB of meaningful data. B contains a sequence of randomly generated bits to be used as a one-time pad for encryption. I use B to encrypt the data on A, store this data onto C, and then destroy A.

B and C now can be combined to retrieve the information that was once stored on A. But B and C by themselves are just random sequences of bits. Where is the information, and therefore mass, of A, which still exists in B and C? If it is in one or both of B and C then I can destroy the information (and therefore reduce mass) in one by destroying the other.