I have my mac Mini with an external HD attached via Thunderbolt. I use to have two different OS, one in the internal HD and one on the external (eg Catalina and Big Sur). Will installing LR on both OS be considered as two computers? I also need to use LR and PS on my MacBook Pro.
the only (insignificant for internet connect computers) limitation is that you can sign-in (and therefore start your adobe programs) on, at most, two computers at any one time. because you can sign out from either (or both) signed-in computers using a 3rd computer, this could not (imo) be easier.
and as @Peru Bob stated, two different hard drives will be recognized by adobe as two different computers even if they're both installed on the same computer.