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SFSCQ is the first file system with a machine-checked proof of security. To develop, specify, and prove SFSCQ, this paper introduces DiskSec, a novel approach for reasoning about confidentiality of storage systems, such as a file system. DiskSec addresses the challenge of specifying confidentiality using the notion of _data noninterference_ to find a middle ground between strong and precise information-flow-control guarantees and the weaker but more practical discretionary access control. DiskSec factors out reasoning about confidentiality from other properties (such as functional correctness) using a notion of _sealed blocks_. Sealed blocks enforce that the file system treats confidential file blocks as opaque in the bulk of the code, greatly reducing the effort of proving data noninterference. An evaluation of SFSCQ shows that its theorems preclude security bugs that have been found in real file systems, that DiskSec imposes little performance overhead, and that SFSCQ's incremental development effort, on top of DiskSec and DFSCQ, on which it is based, is moderate.