I have a MacBook Pro (mid 2009, El Capitan) that has two, physical, internal hard drives in it—I removed the built-in dvd drive and replaced it with a second HD.
I've installed Ubuntu 20.04 on the second drive and am able to boot into either system without, as far as I can tell, any problems.
Since I am quite new at using Linux, I'm a little gun-shy about accidentally doing something stupid and screwing up the macOS install on the other HD (yes, I have backups). Is there a way to permanently prevent Ubuntu from even knowing that the second HD exists at all, so that accidents like, "oops, I meant sda1 not sda2!" can't happen. Like a permanent quarantine, you might say.
During the entire installation and configuration process, I physically had the macOS disconnected so that there was no danger but, this is a little inconvenient for booting back and forth not to mention taxing on a 12 year old piece of hardware.
*Yes, I understand that there are never 100% guarantees when an ignorant user, like me, is swimming in dangerous waters but, you know what I mean.