You can always up date the MB drivers, the USB drivers included. Does your external drive have drivers for it?
Maybe it is the partition format that is not recognized.
Edit:
This might help.
Open Device Manager. A list of available matching items appears above the search box; click Device Manager. Device Manager opens with your computer name at the top and a list of devices that are installed on your computer beneath your computer name.
Locate the malfunctioning device in the list of devices, right-click the device, and then click Uninstall. When a warning message appears that you are uninstalling a device from your system, click OK. Device manager uninstalls the device.
Right-click your computer name in Device Manager and click Scan for hardware changes. Device manager scans your system and automatically reinstalls your device.
Right-click the device and click Properties. On the General tab of the device Properties dialog box, in Device status, you should see the message This device is working properly.