The dedibox page says “en le démarrant en réseau sur un mini système d’exploitation”, which means that your server will be rebooted, on the network, to a mini-os. I can’t test right now (I would need to reboot my server :), but I think this is exactly what happens: you pxe boot a small linux distro, and get access to all your partitions. And you don’t have any time limit :) If you need some help translating parts of their pages, just ask me.