The invention relates to a mobile data storage device (1) with high storage density, comprising a flat support layer (3) with a first (10) and second (5) flat side, a communication connection (2) with a contact section (9) and at least one semiconductor storage device (4), which is designed as a non-volatile read-write memory. The support layer (3) is arranged with a section of the first flat side (10) on a mounting surface (11) of the contact section (9) and the mounting surface (11) is offset relative to the middle plane (22) of the communication connection (2) in a direction perpendicular to the middle plane (22). Furthermore, the invention relates to a pocket-size tool or tool card with a data storage device.