An external harddrive is a great way to back up all your system files but you may of attached one to your computer and found that it wont boot and hangs on the bios information screen.
To get around this unplug the power on the USB External harddrive and then reboot - when your computer turns on you need to enter the bios - to do this hit the del key when you see the screen that shows your post screen.
In the bios you need to find USB mass storage support and enable this - exit your bios saving the change this normally is by hitting the F10 key. Then say yes to save changes.
Reboot your computer with your USB external Harddrive plugged in and it should boot you in to windows.
Update: My computer stopped booting and again was haning on the BIOS screen - I unplugged my USB drive and moved it to another USB slot and rebooted this fixed the problem.
