USB power is not that reliable. While those cables that connect to two USB ports seem to fair better you still see your occasional failure on some machines.
Will yours work? No one can answer.
Looking to get a WD Passport to take my music and such with me while travelling (without the hassle of the power adapter for the external HD), but it looks like some people have noted issues with transfering large files/directories. Is this just rumor or is it really an issue? Most of the use would be backing up My Docs, music, and video folders, so fairly large directories.