plug them both in the first one you pluged in will work the second one will give you a signiture collision.
Go to control panel, admin tasks, computer managment, disk manager, there you will see both your disks, one healthy one offline.
Right click on the little yellow ! and click online. At that point windows will give it a new signature. Keep in mind if it is bootable it may no longer boot. But chances are your not booting from a usb disk.