> Using 32–bit apps with Big Sur
I recently upgraded to Big Sur, but have now discovered that I need to use an old 32–bit app. What’s the best way to be able to run Mojave as well?
There are two good solutions, depending on whether you want simultaneous access to both the old app and your current ones. If you do, then you should run Mojave in a virtual machine from Parallels Desktop, VMware, or another virtualizer. This is quick to set up, and simplest in Parallels Desktop, which will install direct from a downloaded Mojave installer app, as listed at bit.ly/ mac371macoslegacy.
If you don’t need that degree of access, but just want to be able to start up in Mojave occasionally, then you can install a copy on an external drive, or even, if you prefer, on a volume on your internal storage.
Recent versions of macOS won’t let you run older installers such as that for Mojave, though. To do that, you must make an external bootable installer, using a USB flash drive, or a partition on an SSD, perhaps. Apple provides full instructions at bit.ly/ mac371macosinstall. You can then set up your old app and anything else you need there, and switch between macOS using the Startup Disk pane. One snag with this is that you’ll find that Mojave doesn’t understand Big Sur at all, so you must avoid using any of its tools to alter anything on your Mac’s internal storage, apart from copying the files which you need.