First I would boot from the external drive to be certain that it is bootable before erasing the SSD on your Mac. But if you are confident your clone is working then you can safely erase your SSD, but that may not be necessary. Disk Utility in Sierra can non-destructively partition a drive in the free space on that drive. (but I would never attempt that without a known good bootable clone in case Murphy's Law kicks in.)
If you are going to install Catalina anywhere: Power down your computer and wait 10 seconds then start your computer while holding down Option+Command+R (⌥⌘R) until you see the Apple logo appear on your screen. This will boot your computer from an internet Recovery Drive and from there you can install Catalina on your SSD or any drive partition/volume that can be formatted APFS. If you are installing it on your SSD the installer will non-destructively reformat your SSD to APFS.
APFS is Apple's new file system that replaces HFS+ (MacOS extended) which has some really wonderful and amazing features such as multiple volumes on the same drive or partition and each volume can expand to occupy the entire physical capacity of the partition or drive. In many ways it is a mind bender. But the fact is Mojave can run on APFS or HFS+ while Catalina will only run on APFS but it can access files on an HFS+ volume. NOTE: APFS is highly optimized for use on an SSD but it will also work on an HD.
Volume is probably a new and confusing term for you but technically every drive or partition on a drive has/is a volume and the files are contained in that volume. With APFS a drive or partition can have one or more volumes and a volume may contain sub volumes. Confusing? You bet! it took me a while to wrap my brain around all of this but it offers amazing flexibility and features. It helped me to think of a volume as a .dmg on steroids which turns out to be a workable analogy.
Last edited by joemikeb; 04/15/2007:02 PM. Reason: boo boo
If we knew what it was we were doing, it wouldn't be called research, would it?
