After Security Update 2018-001, MacPro 1,1 no longer boot

[r2d3]

My MacPro 1,1 using Piker boot.efi worked fine until the last "Security Update 2018-001".

Now, the machine starts booting then loop back to chime ...

In verbose mode, no "interesting" (for debug) message seems to be shown.

Any idea ?

I had the same problem (see this thread) and was able to recover by restoring my last full backup.

Alternately, some people on that thread have been able to simply re-install their 'pikified' USB install image to the broken target disk to recover, preserving user data and settings.

Going forward, I logged in as 'admin' and set that Security Update 2018-001 to be 'hidden' and will be sure to check that macrumors.com thread before applying any other kernel updates in future.

[b23prodtm]

Get another Mac and plug in the HD in it (external is) we need to copy boot.efi into /System/Library/CoreServices/. and /usr/standalone/i386 because it was overwritten by the security update. That’s it !
Source:https://www.lecafeduretro.fr/2019/09/installer-mac-os-x-el-capitan-sur-un.html?m=1