A USB hard drive is a portable external storage device that works just like a conventional hard drive. The device is simply plugged into a USB port on your computer as you would connect a flash drive. Unlike a flash drive, however, a USB hard drive can store a much larger amount of data. If your computer will not recognize your USB hard drive, you can try troubleshooting steps to resolve the problem.
Plug one end of the USB cable into the port on your hard drive and plug the other end into the USB port of your computer. Ensure that both ends of the cable are connected securely. Or even get a usb 3 lead. USB 3 card will provide more power when needed to the device to gain maximum throughput and conserve power when the device is connected but is idling http://blog.premiumusb.com/2010/09/usb-2-0-vs-usb-3-0/
Open "My Computer" (Windows XP) or "Computer" (Windows Vista/7). The USB device should be listed as "removable disk" once it is plugged into the computer. If it is not listed among the available drives on your computer, proceed to the next step. Ensure that the data cable is working properly. Sometimes they become faulty due to overstretching age or heavy objects sitting on top of them. Try another USB cable if you have one available. Disconnect all other USB devices from the computer as they may interfere with the operating system detecting the external hard drive. Reboot the computer if the USB hard drive is still not recognized. Plug the USB cable into another USB port and check to see if the device is detected. If you are using a USB hub, remove the device and plug the external hard drive directly into a USB port. If the computer still fails to recognize the device, the external hard drive itself may be experiencing a hardware problem. Check the status of the hard drive in the "Device Manager" window. Under Windows XP, right click "My Computer" and select "Device Manager." For Windows Vista and Windows 7, click the "Windows" button in the lower left corner of the screen, then type "Computer Management" in the search bar and press "Enter." Select "Device Manager" in the "Computer Management" window. In the Device Manager window, click the "+" sign that appears beside "Disk Drives." If the external drive is not listed, click "Other Devices" in the Device Manager window and see if an "Unknown Device" appears. Windows 7 First, please disconnect all USB devices and then perform the following steps:
Click "Start" button, type "devmgmt.msc" (without quotation marks) in the "Search" bar and press "Enter". Click "Continue" if necessary.
In Device Manager, double click to expand "Universal Serial Bus controllers", right click on the Host Controller, click "Uninstall" and click "OK".
Repeat the step 2 to uninstall all items under "Universal Serial Bus controllers".
Then, restart your computer and Windows 7 will reinstall all USB controllers automatically. If the hard drive that has failed is the same one that you are trying to recover from might the reason that you are getting this error. This could be an issue where the part of the hard drive you are trying to access is corrupt and not accessible which would explain the reason that it would let you access the other partition. Now I have seen cases where Malware can break the link of a Hard Drive. So let's try a couple of steps. Let's go back in to "Disk Management
" Lets right click on the partition that you are not able to access and select "Change Drive Letter and Paths
" Now try selecting a new Drive Letter. This will restore the path if it has been broken. The external hard drive may have been detected but the device is not working properly. If it is still under warranty, take the device to the computer store where it was purchased to have it repaired or replaced.