OK, it looks like another discussion of updating to a new version of the Mac OS versus doing a clean, fresh installation of it. I am going to explain, again, my procedure for doing a fresh, clean installation of a new (or newer) version of the Mac OS.
First of all, no matter which approach one takes, it is imperative to already have a recent backup. I cannot stress that enough. Myself, I always do 2 backups every week for each of my Macs to external "devices" (actually partitions, which I describe below).
Secondly, and again this applies to no matter which approach one takes, it is also critical to insure that any and all third party software one uses is compatible with the new Mac OS one is "moving to". Again, cannot stress how important that is.
Third, before beginning, and at least because I use the "clean installation" approach, it is imperative to 1) have already downloaded the full installation file, 2) make a copy of it to another location on your Mac, and 3) remove the one inside the Applications folder.
OK, I am now going to describe what I do. I am "speaking" from the aspect of using SuperDuper! (SD) as my backup software, and this same procedure is also applicable for people that use the other excellent program, Carbon Copy Cloner (CCC). I cannot speak for Time Machine, as I have never used it. The distinct advantage of either SD or CCC is that the backups each of them make are bootable. Let me repeat that: such backups are bootable. That means that after the backup completes, one can boot their machine from such a backup. That to me (and I suspect others) is a huge feature and advantage of either SD or CCC over Time Machine.
One other thing to my case: My Mac Mini has a 256 gig SSD, and my MacBook Air has a 252 gig SSD. For both of those machines, I am using less than 90 gig of space. Hence, for the external SSDs I am backing up to, each of them are 512 gig in size, but I have 3 partitions on each of them. Two of those partitions are about 110 gig in size, which I use for each of my backups for each of my machines. SD (and I suspect CCC) are "smart" enough to handle the backups. That is, even though the total size of the SSD I am backing up from is 256/252 gig, SD just "knows" that it is backing up less than 90 gig, and thus can backup to a "device"/partition size of 110 gig. Obviously, if the source device I am backing up from is using more than 110 gig in space, the "associated" partition on my external SSD would need to be larger than 110 gig.
OK, I can FINALLY start describing my procedure:
1. Use Onyx and TechTool to do one final disk cleanup and maintenance of my current Mac OS system (right now, that is Mojave).
2. Use SuperDuper! to do the 2 backups to the respective partitions on my external SSDs. Remember, this is backing up EVERYTHING on the respective Mac to that external partition.
3. Restart my Mac from that just completed SuperDuper! backup. Again, remember that such backups are bootable.
4. Select Disk Utility there (ie, on the backup), and use it to Erase and Format the internal SSD on the respective Mac I just backed up from, in step 2 above.
5. Navigate to the location of that downloaded full installation file of the Mac OS I am "moving to" (I mentioned that above).
6. Launch that file, and do a clean, fresh, virgin installation of that Mac OS onto the internal SSD of the respective Mac.
7. Once that installation completes (and the installer does this), I am offered the opportunity to migrate "files" (ie, Applications, Data files/folders, Settings, etc.) from either another Mac, or a backup. So, I just point the installer to that just completed SuperDuper! backup to obtain all that "stuff", and away it goes.
8. Once that completes, I restart my Mac from the "new Mac OS" internal SSD on the respective Mac.
9. In my particular case, I then have 2 additional things to take care of:
A. For the excellent program Onyx, Titanium software develops Mac OS-specific versions of it. So, I would remove the "prior OS" version fo Onyx, and install the new one.
B. For TechTool Pro, I need to set up an eDrive partition. That partition is needed to use a lot of the features of TechTool Pro.
That might seem like a lot of work and effort, but I have been doing it for so, so long, that it is second nature to me. Also, I ALWAYS wind up with a stable system.
OK, I am going to leave this as is, and am not going to comment on just upgrading a present Mac OS to a new one. Suffice to say that for some folks, such an approach works fine, but if i were to do it, I would not do it for more than one of two prior versions of the Mac OS.
