Catalina / Mojave conflict in dual boot machine

[BlueMacawBird]

The background is that I have an iMac 18,3 running Mojave. I need Mojave since I have too many 32 bit programs to leave behind. But there are some things that need a later OS, so my solution was to get an external TB3 SSD and use that as a boot disk, to be selected when needed. I did a clean Catalina install on the TB drive and migrated the contents of the internal device (also an SSD). The install and migration seemed to go fine and the external SSD on TB3 is almost as fast as the internal SSD. While in Catalina I decided to delete the old programs on that drive that were incompatible, and got rid of them. Later when I rebooted into Mojave I was astonished to find that those files had been removed from the Mojave boot volume as well. I was able to recover them to my Mojave volume from my TM backup. Returning to the Catalina volume I found them back there as well. I have never heard of this behavior where changes made to one independent volume were reflected on an unrelated volume. One of the programs I wanted was a 40+ GB game, and when I installed it on the Catalina volume it appeared on my Mojave volume as well. That's especially irritating since the program cannot run and simply takes up a useful amount of space.

All the steps I took during the install were familiar to me and I did not see anything unusual. However, looking at the usage figures for the Catalina volume it appears that the install did not actually move that many of the files from my Mojave volume as it should have. Only about 250GB of 650GB are actually on the Catalina volume, although it is confusing to try to compare them because of the way storage is now shown.

I wish that I had cloned Mojave to the new SSD, and then installed Catalina on it. I've done that to other external devices before and had no problem. That will be my next step, but I would like to know what is going on before I do that. I'm somewhat concerned that the Mojave volume may now be linked to the new Catalina volume, and erasing the current Catalina copy may cause damage to the Mojave volume ( I can do a basic test on that by ejecting the Catalina volume and seeing if the machine complains, but it may be more subtle than that).

So if anyone can explain this I would appreciate it.

Thanks

John

 

[BlueMacawBird]

The update is that I gave up on the installation that I had migrated from the internal SSD, and erased the TB SSD. I then updated an external HD clone, ran the Catalina installer from that onto the TB SSD, and migrated the cloned copy to complete the process. The copy of Catalina now works properly and does not do anything to the internal Mojave SSD. So now I'm good, but there is no explanation for what it was doing the first time around.

John

 

[brianmowrey]

I'm curious, before the reinstall, you did replicate the behavior at some point? If it only happened once, how can you be sure it wasn't just a matter of accidentally being in the Mojave drive when you deleted the apps?

 

[BlueMacawBird]

I was able to replicate the behavior at will, carefully noting which files had been removed from the Catalina drive and then moving to the Mojave drive and finding the changes made there as well. It worked both ways, with deleted items on Mojave also being deleted on Catalina. The files I was having problems with were games, in their own folder. Limited testing in the Applications folder did not show the same behavior consistently, with some files being deleted and others staying on one of the drives. It was very strange and I have no idea what was going on. Luckily I noticed it right away and was able to do the reinstall before significant damage was done. It sounds paranoid, but the reason why I did the reinstall from an external HD was to avoid any possible linkage between the internal SSD and the TB SSD with Catalina.

John

 

[brianmowrey]

Thanks for clarifying. Now I can fully appreciate the spookiness of your experience.

The one culprit I could pin such a quirk on would be T2 chip somehow revoking permission for the apps. (But you would expect the apps to still show in Finder, just require regranting permissions. Still, it would be a hypothesis.) But 27" iMac didn't get T2 until this year, so that's out!

 

[BlueMacawBird]

No T2 chip in any of my machines. I'm not a fan of the T2, but sooner or later will have to learn to live with it. I recently saw that there is a vulnerability that has been found in the chip, that cannot be patched. Hopefully Apple fixes that in the upcoming machines.

John

 

[brianmowrey]

I recently saw that there is a vulnerability that has been found in the chip, that cannot be patched.

Yes, but not yet to a point where anything can be planted into RAM that would withstand a reset. Basic EFI was always secure enough to make social hacks a lower friction pathway to your data than physical hacks anyway. T2 and its successors are all pointless.