We present fat12-engine, a minimal, dependency-free FAT12 read/write layer that brings an atomic verify-after-write and rollback contract to removable and embedded media on which the operating system provides no filesystem stack and no journaling. Each multi-step operation captures a RAM-resident undo-log of every touched sector, reads back and byte-compares every data cluster after writing it, commits the FAT allocation before writing the directory entry that makes the file reachable, and on any failure restores the volume to a byte-for-byte identical pre-operation state. The named artifact is the pure-JVM :core engine (zero Android or libaums dependencies); an Android USB-OTG application is an illustrative demonstrator only. We evaluate crash-consistency by deterministic fault injection, including byte-for-byte rollback tests, and validate conformance against mkfs.fat golden images plus an independent fsck.fat -n oracle. The engine targets a removable-FAT12 niche unserved by existing (FAT32-only) Android tooling