A carriage with a cell carrier engages and retains a vertically driven cell so that the cell and its contents, such as a data storage cartridge, may be moved laterally from one linear library to another. A portal engagement device of the cell carrier includes a movable rack so that a pinion of the cell may move up or down the rack, and has pins that, when the device is down, engage a library vertical portal having a fixed rack. A cam state machine of the cell carrier guides a spring loaded cam follower hook of the cell in a pattern to lock the hook in position and, when hooked, attempted downward movement of the vertically driven cell moves the portal engagement device upward to disengage the pins of the portal engagement device from the vertical portal. The carriage may then transport the cell to another vertical portal.