That sometimes happens when the OS sees the device, but cannot read it because the data on it is corrupt or the drive is not formatted.
Does the drive work fine on a different computer? Have you tried a different USB port?
If the drive is good and readable on another computer, I would remove it from this computer, then do a "cold" reboot. By “cold”, I mean shut down the computer and flip the master power switch on the back of the power supply (if your supply has one) to off (or “0”) or unplug the power supply from the wall for about 15 seconds. Then turn the master power switch to on (or “1”) or reconnect power and boot up and see what happens. By totally removing power, you remove the +5Vsb standby voltage all ATX Form Factor power supplies are required to supply whenever the power supply is plugged into the wall and (if applicable) the master power switch is set to on. This standby voltage is distributed throughout several points on the motherboard, including RAM for faster boots and the USB ports too. This also keeps alive several features, including “wake on keyboard” and “wake on mouse”, and the network interface for “Wake on LAN” commands. A cold reboot ensures any device settings that may be held by the standby voltage are released, then reset when power is restored.
On a notebook, a "cold" reboot requires a complete shutdown of Windows, closing the lid, unplugging the charger and removing the battery for about a minute. Then reversing the steps to boot.
