The ornamental design for an externalstoragedevice, as shown and described.
FIG. 1 is a perspective view of an external 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 plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a bottom perspective view thereof.
The broken lines are for environmental purposes only and forms no part of the claimed design. The shade lines in the figures show contour and not surface ornamentation.