I want to upgrade to Ventura, but I know that a few of my software programs will require Catalina, which I am on now, in order to work. How should I proceed so I can go back and forth as needed?
you can use internet recovery. first make sure to erase and format the external SSD as APFS. please see Erase and reformat a storage device in Disk Utility on Mac - Apple Support.
then please see How to reinstall macOS - Apple Support (CA) and when you select "reinstall Catalina" choose the external drive as the one you want to install Catalina on.
make sure the external Catalina install is working for you and then you can go ahead and upgrade your local drive.
you can use internet recovery. first make sure to erase and format the external SSD as APFS. please see Erase and reformat a storage device in Disk Utility on Mac - Apple Support.
then please see How to reinstall macOS - Apple Support (CA) and when you select "reinstall Catalina" choose the external drive as the one you want to install Catalina on.
make sure the external Catalina install is working for you and then you can go ahead and upgrade your local drive.
I too have an intermittent need to use Catalina. so I keep my local drive up to date, (currently Ventura) and I have installed Catalina on an external Samsung T7 SSD. I keep my local drive as my designated startup disk. and when I need to use Catalina, I simply restart my Mac then holding my keyboard's "option" key while starting up until I see the screen that allows me to choose which drive I would like to start up with.
It sounds like the SSD would have to be dedicated to just holding Catalina. Is that true? I have need to use Mountain Lion as well and have been using Parallels Desktop for that on my machine running Catalina. I'm wondering if that work-around will become a problem when I move to Ventura. Could I also mount a version of Mountain Lion on the same SSD or if I would have to use a separate, dedicated SSD for that OS and avoid Parallels altogether? Thanks again. Much appreciated!
sorry, but frankly, I just don't know. I would think that you could possibly partition the external drive and use it for both Catalina and parallels with mountain lion each on it's own partition. but I would have no clue on if that is possible or not as I have never experimented with parallels to understand the inner workings of it. if you can install mountain lion onto an external SSD using parallels I would dedicate a separate external SSD to each of those macOSes. but again, I'm over my head with this aspect of your question. maybe someone else will weigh in on this.
Thanks for the reply. This sounds perfect. How would I install Catalina onto an SSD to make it a bootable drive?
How can I maintain access to Catalina after I upgrade to Ventura?
