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In the end of 2022 in Russian Federation, recommendations for standardization were adopted defining a block ciphers mode of operation for block-oriented storage devices protection. This mode is called Disk Encryption with Counter (DEC). We study the DEC mode security properties in post-quantum setting. We define a notion of stateful tweakable encryption scheme and show that the simplified DEC mode instantiated by a block cipher satisfies this definition. Further, we define provable security models from Q2 class formalizing the notion of information confidentiality on a block-oriented storage device, when an adversary could perform superposition queries to encryption oracle. We establish a relationship between introduced security notions and then we show that the DEC mode is theoretically insecure in these models.