One of the rarest creatures in the world is a two-headed snake, although they can be found in places like the San Diego Zoo. When pondering a slipped USB cable, keep that snake in mind. Such a cable has a single type B connector on one end and two standard USB connectors on the other end. You might be confused about the two-headed end of the cable, but it's really all about funnelling more power to certain portable hard drives.
Feel the Power
Some computers, particularly older machines, have underpowered USB ports. If you try to connect one of the newer portable hard drives via an older USB port, the computer may not recognise the device. You won't see it as one of your drive choices. One way to fix this problem is with a slipped USB cable, more commonly called a USB Y-cable. Simply plug the single, mini-USB connector into the portable hard drive and then plug both USB connectors into two USB ports on the computer. It doesn't matter which head you plug in first. You should now have enough power so that the hard drive becomes usable.
Two Types of Hard Drives
Portable hard drives come in two distinct classes. "Portable" and "external" are sometimes used interchangeably, but If you look closely at store shelves you'll see that "external" hard drives are not completely portable. You have to plug them into an AC outlet. One good thing about external drives is that they get their power that way. Portable hard drives lack an AC power cord and get their power from the computer's USB port only.
Check the Drive
After you plug in both of the hard drive's USB connectors into the computer, check to see if it made it. Go to "Start," "Computer" and confirm that the portable hard drive is now there. It will usually be called the "D, "E" or "F" drive, depending on what else you have plugged into your machine. Double-click the letter to open the hard drive. Once open, right-click an area on the drive and select Properties. Check that the "Free Space" area coincides with the actual space left on the drive.
You can find some USB 4.0 devices, but the new kid on the block has not really arrived yet. You'll mostly find USB 3.0 ports on new computers, with USB 2.0 also hanging around. Speeds can vary depending on devices and other issues, but the USB 3.0 transfer rate is around 4.8GB a second compared to USB 2.0, which is around 60MB per second.