O Debian-Update.iso não é inicializável você precisa baixar uma imagem inicializável de aqui .
Quais são os CDs / DVDs de atualização?
Note that update CDs/DVDs are not meant to boot, they just contain the packages needed to upgrade an existing installation. If you don't have that existing installation, then you'll need to use the normal installation CDs/DVDs. After the new system is booted, the updated CD/DVD can be added with
apt-cdrom add
.