Complicated boot situation: Unable to go to Mac

[Strategia]

Hello,
After installing Linux Mint on my 2013 iMac I'm unable to return to macOS on my external SSD (where I want to end up in), macOS on my internal HD, or anything else really. When I start the computer GRUB takes over, and I am unable to get to the bootloader screen by pressing alt. Somehow GRUB is affecting that. I was thinking of trying to install Refind but I think System Integrity Protection would mess up that and leave me with no OS to boot off instead of just Linux. Any suggestions or help would be appreciated.

 

[Strategia]

By resetting the PRAM, which reset the startup disk choice and thus avoiding that pesky GRUB, I was able to return to macOS on my external SSD just in time for the release of Catalina. Unfortunately, Linux and macOS on my internal hard drive were rendered destroyed and unbootable in the cross-fire. Well, at least the 1 TB of space on my internal drive can be repurposed for something else.

 

[Roxy.music]

I had the same thing about Grub it did go away after a time it is something to do with Linux when i tried to get Linux on a external drive.
i have the problem of my Windows 10 on a T5 .SSD it does not show on start up discSomeone did mention PRAM how do i get to PRAM on here ?I up dated to Catalina and had the problem with Windows not showing up on the start up on system Preferences it worked before i up dated to Catalina

 

[Roxy.music]

I looked up PRAM and did what they said Reset NVRAM or PRAM on your Mac Shut down your Mac, then turn it on and immediately press and hold these four keys together: Option, Command, P, and R. You can release the keys after about 20 seconds, during which your Mac might appear to restart.
On Mac computers that play a startup sound, you can release the keys after the second startup sound.When it was done I got that GRUB. back again.