Techie question

[desperance]

So I have bought a portable external hard drive, for backy-uppy purposes: but I am a little bewildered. In total denial of the official printed instructions, which feature only a common-or-garden cable that plugs into the hard drive at one end and a USB port at the other, the actual cable that came with has one end that plugs into the device, a USB port that plugs into the computer - and then another USB port connected in series. All on the one cable. Does this mean that I can plug two computers into the same device? And if not, what the hell is the point of this...?