I completely agree with everything posted above, but I don't see evidence that Apple intends to stop 32-bit apps in NVRAM for normal users. Apple's statement on 32-bit apps is that macOS 10.13 is the last to run them "without compromise", which says to me that MikeRose's approach is reasonable. I don't see any reason that he won't be able to maintain a bootable drive with the last version of macOS 10.13 High Sierra to continue to run 32-bit apps "without compromise".A lot of unknown factors here (which Mac, which OS version on that Mac, which external drive etc.) make an accurate determination difficult. All of those factors will need to support an external boot of a (presumably) prior macOS version. Plus, Apple's macOS 10.13.4 High Sierra SDK Release Notes (Howard Oakley's site also has a nice description) demonstrate how Apple can stop 32-bit apps in NVRAM, which, though surmountable, is yet another obstacle.