Windows 7 installation via an external usb dvd drive

Hello everybody !

So here is my problem :

I have replace my superdrive by a SSD disk and re instal Lion on it 😉 I steel got my orginal hard drive and I use it to save all my documents ... From here everythings it's ok but ...

I need to instal windows 7 ultimate 64bits on a partition located on the hard drive, I use the boot camp assistant, download bootcamp 4, create a partition on my hard drive ok ok, and I put my DVD of Win 7 into an usb external DVD drive because I have replace my superdrive by the SSD !

But when the mac reboot, the DVD start to turn and suddently stop and the apple logo alternate with an image of a folder with an exclamation point on it like it seems to not found the DVD to boot on it !

What am I suppose to now ?

Thank you.

And sorry for my english if I made some mistake. I'm french 🙂