Getting around BTX halted after update

After the last update server crashes with BTX halted.

I'm on 12.0-CURRENT 64bit and running a Supermicro X9DR3 with dual Intel CPU's and 48G RAM with two ZFS mirrors, One mirror for the O/S and one for user space. The first mirror is on SSD's and the 2nd is on spinning SATA. There is no WHEA in BIOS to disable. It has been working since October last year. On previous failed updates I could always choose an earler boot environment and get around it, but here I have no idea.

The boot: prompt fails on loading tank1/ROOT/12.0-CURRENT-up-20170402_195554:/boot/zfsloader and returns a register dump[?] and BTX halted. It seems I should be able to pick another one but I don't know how to list it at the boot: prompt. BTX has a number of commands but I've not been able to find anything on that so far.

The other BTX errors on the forum does not appear to apply to my situation, but I'm quite new to BSD so I could be wrong. Any tips will be appreciated!