Try browse the device manager, you'll see that there is a device (mass storage or IDE bridge.. or sounds something like that). try to install/upgrade the driver. When it asked the location of the source driver, just type c:/windows/inf and it'll try to complete installing. This will lead you to two options, if the usb mass storage is working fine, you'll get the drivers installed and completed beautifully, but there are some cases that the mass storage device drivers is locked/disabled (code 22). just follow the steps provided by the hardware installation wizard that you are on, it'll do the magic to make the ext hdd working again on your PC.
Some External HDD was preconfigured to run on specific data connection channel, that will give conflicts if it use the same channel as other installed usb devices. CMIIW..