Making Bootable USB Custom Installer with site69.tgz

Hi,

I want to create a custom OpenBSD installer, so I prepared site69.tgz.
However I couldn't create a bootable disk image.

Firstly, I mounted the OpenBSD installer;

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vnconfig vnd0 install69.img
mount /dev/vnd0a /mnt

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And I created my own installer image;

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dd if=/dev/zero of=disk.img bs=1m count=4096
vnconfig vnd1 disk.img
fdisk -i vnd1
echo "a\n\n\n\n\nw\nq\n" | disklabel -E vnd1
newfs /dev/rvnd1a
mkdir /mnt2
mount /dev/vnd1a /mnt2

cp -r /mnt/* /mnt2/

cp site69.tgz /mnt2/6.9/amd64/

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Made disk image bootable;

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installboot -r /mnt2 vnd1 /usr/mdec/biosboot /usr/mdec/boot

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Unmount disks;

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umount /mnt
umount /mnt2
vnconfig -u vnd1
vnconfig -u vnd0

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Lastly, I write this image to a USB.

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dd if=disk.img of=/dev/sd1c bs=1m

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When I try to boot from this USB, I got the following message;

MBR on floppy or old BIOS
Using drive 0, partition 3.
No O/S

Why am I getting this error and how can I solve this problem?

Thanks..

Ref: https://www.undeadly.org/cgi?action=article;sid=20140225072408