The ornamental design for a “portable storage device,” as shown and described.
FIG. 1 is a perspective view of a “portable storage device” showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a perspective view showing the practical usage thereof.
The broken line circles immediately adjacent the solid line circles represent the bounds of the claimed design while all other broken lines are directed to unclaimed portions and are for the purpose of illustrating unclaimed subject matter only; all broken lines form no part of the claimed design.