Create a partition table and make a fat32 partition that uses the whole usb key space.

cfdisk /dev/sdb mkfs.vfat /dev/sdb1

Install grub on the usb key

mount /dev/sdb1 /mnt grub-install –no-floppy –root-directory=/mnt /dev/sdb

Create a grub.cfg file in /mnt/boot/grub/ with the following contents:

menuentry “debian-installer-i386” { linux /boot/vmlinuzi386 vga=788 – quiet initrd /boot/initrdi386.gz }
menuentry “debian-installer-i386 expert” { linux /boot/vmlinuzi386 priority=low vga=788 – initrd /boot/initrdi386.gz }
menuentry “debian-installer-amd64” { linux /boot/vmlinuzamd64 vga=788 – quiet initrd /boot/initrdamd64.gz }
menuentry “debian-installer-amd64 expert” { linux /boot/vmlinuzamd64 priority=low vga=788 – initrd /boot/initrdamd64.gz }
menuentry “live amd64” { linux /boot/liveamd64/vmlinuz boot=live live-media-path=/boot/liveamd64 persistent-path=/persistent/amd64/ live-config persistent initrd /boot/liveamd64/initrd.img }
menuentry “live i386” { linux /boot/livei386/vmlinuz boot=live live-media-path=/boot/livei386 persistent-path=/persistent/i386/ live-config persistent initrd /boot/livei386/initrd.img }

Get the hd-usb debian installer vmlinuz and initrd.gz files from the web. Rename them to vmlinuz$arch and initrd$arch.gz, where $arch is the computer architecture (i386, amd64). Copy them to /mnt/boot.

Get the corresponding netinstall iso files for the installed architectures and copy them to /mnt