Try other cable to connect the ex-hdd to the computer , i think the cable might be dammaged .

External hard drives are plug-and-play devices used to store music, video and data files. Once the drive is connected, the computer checks its systems for the appropriate drivers, loads them, and the external drive is ready to use. If the computer does not recognize the external drive, plug it into another computer to ensure the drive is not corrupted. If the drive is good, a few simple steps should help re-enable the drive for use.
Switch the power button to "off." Disconnect the power cord from any extension cords or power strip, then re-seat the power cord into the hard drive. Connect the cord directly into an outlet and power up the drive. If the drive is powered and still not working, check the data cable. Power down the hard drive and disconnect the data cable from the computer. Wait 30 seconds to one minute. Reconnect the data cable and power up the drive. If the drive is not detected, power down the hard drive and connect the cable to a different port. Power up the drive. If the drive is detected but not working, check for a missing driver. Click on the "Start" button, right-click "Computer" and select "Manage." Enter your administrator password if requested. Select "Device Manager" and double-click "Disk Drives." Right-click the yellow exclamation point and install the missing driver. Close the Device Manager. If the drive is detected but not working check to ensure it has an assigned drive letter. Disconnect all devices connected by USB cables from the computer. Reconnect the drive's data and USB power cables directly into the computer. Click on the "Start" button, right-click "Computer" and select "Manage." Enter your administrator password if requested. Click on "Disk Management," right-click the external hard drive, click "change drive letter and path," and click "Add." Assign a drive letter. Avoid using A, B or C (C is usually assigned to the computer's internal hard drive). Click "OK" close the window. If the drive is detected but not working, check whether the drive is in sleep mode. Disconnect all devices connected by USB cables from the computer. Reconnect the drive's data and USB power cables directly into the computer. Click the "Start" button, right-click "Computer" and select "Manage." Enter your administrator password if requested. Select "Device Manager" and double-click "Universal Serial Bus Controllers." Double-click the first instance of "USB Root Hub" or "Root Hub." Check for the hard drive. If the drive is not visible, move to the next instance of "USB Root Hub." Continue checking each instance until the hard drive is located. Once located, click the "Power Management" tab and uncheck the "Allow computer to turn off this device to save power" box, click "OK," then close the Device Manager. Power the computer down, wait 30 seconds to one minute, and power the computer back up to ensure the drive is detected. Hope this helps.