You shouldn't need separate vmlinuz and initrd.gz files. You should be
able to boot directly to the iso image. Depending on how talented your
computer's firmware is, you may have to copy the iso image to sdb, not
sdb1. E.g.:
dd if=<iso.image.file>.iso of=/dev/sdX
What instructions did you start with? Exact URL, please.
I've been playing with a separate partition to house a Debian installer
ISO should I ever need it for rescue or re-installation. But that
depends on having grub installed and a suitable entry for the d-i iso
image. If that's what you want to do, I can help with that also.