Quote:
Originally Posted by odamizu
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Also, just to make sure I understand, if I boot from an external drive, it's like having a different and separate computer from my primary Mac? I would create new user accounts and re-install apps, just as if it were a new computer?
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See
https://support.apple.com/en-us/HT202796
Suppose you install two versions of macOS A and B on two drives/partitions respectively.
You can use the same user account, but you don't have to re-install any app that is compatible with both A and B. Otherwise, re-install some apps on an as-needed basis.
If you Mac has USB 3 or some even faster interface, booting from an external SSD should be fast enough. These days SSDs are pretty inexpensive. I tend to keep one for some older version of macOS for some selective apps (similar to your intended use case), and another one for future beta OS for playing around, while keeping the internal SSD as my day-to-day workhorse.