I want to create a boot disk for El Capitan, just to start from so I can use disk utilities like Tech Tool. How do I do this? I see all kinds of instructions online that say how to make a boot disk that installs El Capitan. I just want to start my computer from an external disk -- I am not trying to do a clean install. Is the procedure the same? Will the bootable install disk just startup as normal if I tell it I don't want to reinstall the system somehow? 

It's just that all the instructions seem to assume you want to do a clean install, and that isn't what I am after.
TechTool Pro allows you to create a so-called eDrive. You will find the feature in the GUI choosing Tools > eDrive. You need an external drive (either an external HDD/SSD or a thumb drive). About 15.5 GB of the disk will be reserved to create a new volume "eDrive".

After choosing your system volume as source and a volume on the external disk as destination drive, the external volume will be resized and a new volume eDrive  will be created. All necessary files and folders will be copied to eDrive after starting the task.



Now quit TechTool Pro, open System Preferences > Startup Disk, choose eDrive as boot volume and reboot your Mac.

Your Mac boots from the eDrive and TechTool Pro will be opened and you are free to diagnose your main volume.



Since only about 5.5 GB of the 15.5 GB are used you may install additional tools.