We claim the ornamental design for a mobile drive unit, as shown and described.
FIG. 1 is a top perspective view of a mobile drive unit according to our new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof
FIG. 4 is a first side view thereof
FIG. 5 is a second side view thereof;
FIG. 6 is a rear end view thereof; and,
FIG. 7 is a front end view thereof.
The broken, dashed lines illustrate environment structure and are not part of the claimed design sought to be patented. The dot-dash-dot lines show the boundaries of the claimed design.