It is also important to use the right USB plug at the end of the cable, it may still work with only one connected.

The printed isntruction card that is in the box with the drive shows which of the two can work by itself. The other is only power, no data. It is there "just in case" your computer has USB port that does not supply enough current to run the drive, you attach the second one also, which provides double the current. You may need that and you may not. Also worth trying USB ports on front and back (or side) of the machine as sometimes the computer mfrs make some of them able to supply more current than others. The ones with less current will work okay with lower power things like mice and keyboards but not with hard drives.


--Mac