trying to attach another disk to openbsd72 bhyve guest

Hello, I'm trying to attach another disk to an openbsd72 guest like so:

[snip]
bhyve -c 4 -m 16G -wuH \
-s 0,amd_hostbridge \
-s 3,virtio-blk,/dev/zvol/vms/openbsd64g \
-s 4,ahci-hd,/dev/zvol/vms/openbsd-externalfs0 \ <=== here
-s 5,virtio-net,tap14 \
-s 30,xhci,tablet \
-s 31,lpc -l com1,stdio -l bootrom,/usr/local/share/uefi-firmware/BHYVE_UEFI.fd \
openbsd64g
[snip]

The vm starts fine. I can't see an additional sd* though so can't
initialise the disk. Also tried with virtio-hd same result

Equivalent with freebsd guest is

[snip]
vmrun.sh -c 4 -m 16G -t tap14 -d guest.img -d newdiskimg.img
[snip]

the device would show here as vtbd1. I'm not seeing the sd equivalent with openbsd.
Is this a bhyve issue or an openbsd one?

TIA,