Startup key combinations don't work after disk swap, cannot boot into live installation media

[GalacticLion7]

I recently replaced my old iMac's HDD with an SSD. The SSD had a Linux distribution installed in it, but I didn't bother formatting it because I knew it would be formatted during OS installation.

I think that was a mistake, because I am now unable to get my iMac to boot into my live Linux installation USB. It keeps booting into the Linux distribution that was already in the SSD into a CLI-only "emergency mode" no matter what. None of the Mac startup key combinations seem to work.

Here is the output of efibootmgr -v in the Linux distribution it keeps booting to:

BootCurrent: 0000
BootOrder: FFFF
Boot0080* Mac 05 X
008,0×64028,0×25294648)
PciRoot (8x0) /Pci (Bx1f,8x2)/Sata(0,0,0)/HD(2,GPT,00087fca-363-0000-1859-008022128
BootFFFF*
PciRoot (0x0) /Pci (Ox1a, Ox1)/USB(1,0) /HD(1 , MBR, Bx0, 8x3f ,8xfc7fb9) /File (\boot.efi)

Reordering or deleting the entries didn't seem to fix the issue.

How do I recover from this mistake, preferably without opening the iMac again to access the SSD?

 

[tinygoblin]

Perhaps it's not directly relevant to your case, but remember you should never use efibootmgr on a Mac since it will mess something up (maybe it already did, please do your research, IIRC it at least breaks macOS boot). Use bootoption instead which happens to be alternative developed specifically for Mac.

P.S. I may be wrong and efibootmgr works correctly if only Linux is on the machine's internal SSD. But if there's macOS side by side with it you should definitely stick to bootoption.