how do you do this - a complete virgin install? i am running 10.14.6 and have a lot of junk files from old apps, etc. and would like to do a clean install.
First of all, I use SuperDuper! for my backups to two external Samsung 850 Pro SSDs, and it creates a
bootable backup.
Secondly, here is how I go about this:
1. Download the latest full installation file of the new Mac OS. For example, I will most likely either start with the full OS 10.15.2 or OS10.15.3 full installer file for Catalina.
2. When that download completes, copy that file (in my Applications folder) to another location on my internal SSD. The reason I do this is that when one runs the installer from the Applications folder, it gets removed when the installation completes. I want to have a copy of it, just in case I need it. Most folks
do not do this, and it can cause headaches later on.
3. After that copying, remove the one in the Applications folder. No sense wasting space.
4. Run Onyx, and TechTool Pro, to do final maintenance/cleanup tasks on my present system (OS 10.14.6, in my case).
5. Make SuperDuper! backups of that "clean" system to my external SSDs.
6. Restart the respective machine (I have 2 Macs) from that just completed backup.
7.
On the backup, launch Disk Utility, and Erase and Format the internal SSD on the respective Mac.
8. Navigate to the saved OS installation file (the one i copied in step 2 above).
9. Launch that installation file, and do a clean, fresh, "virgin" installation onto the internal SSD erased and formatted in step 7.
10. When the installation completes, I am afforded the opportunity to migrate/copy needed files, folders, apps, settings, etc. from a backup. I select the just completed SuperDuper! backup that is on the external SSD I have been working from.
11. When that completes, I will be afforded the opportunity to restart my Mac from that "new" system.
12. In my particular case, I have 2 more tasks to need to complete once in my new system environment:
a. Remove the version of Onyx that is there, and install the new OS-specific version of Onyx.
b. Create an eDrive for TechTool Pro.
By doing all that, I have a clean, "mean" new system. Of course, I am doing disk cleanup everyday anyway (most of the time permanently removing deleted EMails I no longer need nor want).
While it does take some time, I have been following that procedure for so long, and it has always resulted in a clean, fresh new system that is trouble free. Of course, I also have insured that all my third party applications i use are compatible with the new OS (or in the case of Onyx, make sure the new version is downloaded, and install in in step 12a above.