Well, I was running my system from a cloned backup - and it crashes after about 15 minutes of usage. I re-imaged the original drive with a fresh El Capitan install. That drive must've gotten fried from the boot-cycling since it crashes after about 1 minute of usage. Running an old hard drive with Lion on it is stable.[/QUOTE
I tried replacing everything that had been changed with no luck. I tried and tried but could not "fix" a system that had undergone the 2018-001 Security Update.
Hi SuperMatt, have you run a re-install to the boot looped drive from a pikified USB EC stick with 10.11.6. It should overwrite your current OS files but leave all your users/data/apps intact. Ive found its the easiest way to "repair" a boot loop drive. You can create new install media and move drives and copy data to and fro if you have another EC capable mac but re-running a re-install from Pike modified USB installer is allot easier, takes less time, doesn't need another mac and restores your system to just before the update without touching a drive. You will though need to be able to boot into 10.6 or above to create a USB stick if you don't have one ready and a boot supported GPU.
Other options are restore from a backup if you have boot support on GPU holding down option key and selecting the install recovery. I have also tested and restored this way but depending on how many GB are on your OS disk the USB route can be much quicker as it replaces only OS files rather than deleting and restoring the disk from scratch as a time machine restore.
Also you could try running the pike app installer in another working bootable OS on the same machine and overwriting the boot loop HD OS. Ive not tested this but it could work for non boot supported GPU situations. Cloned drive backups can also be useful if you have no GPU boot support. Firewire Target disk mode can be used installing from another machine.
Also creating a HD on another EC capable mac but swapping out the boot.efi files (starting again) is an option. Again I've not tested but others have done with positive results on the forum.
Painful but recoverable if your machine can still boot that is. You should backup any important data if you can before reinstalling from a bootable OS or firewire target disk mode. I had a backup so could experiment with my boot drives straight away. A boot looped drive is accessible in the finder from another OS instance to backup data btw (if you caught/stopped the boot loop in time as its apparently crashing HD disks for other users). I only let the machine restart maybe 5 times before realising it was confused. It could go on for a long time if left unattended or started automatically...