@slycat,
Thanks for the pointer for an Arch Linux noob that I am

I did pacman clean, refresh, and upgrade as you've recommended, installed udev-oxnas, and rebooted. udev now works correctly, no more 100% CPU. The upgrade command pacman -Syyuu caused a massive amount of package upgrades

and old udev was removed when udev-oxnas was installed.
-bodhi