Arnaud:

When you get to that message about unusable USB, you are on the right path! You need to read the Wiki more closely.

It says that you then should unplug the usb, hold down the u key on the N900 and plug it back in. Then the N900 will start back up and when you see the usb symbol on the N900 you should stop holding down the U key you have been holding down. Flashing should start.

Read the Wiki for more details, that's where I found this out.

It worked great for me!