Installing Leap 15.3 on an old iMAC. It won’t boot directly after install. Have to hold down the Alt key and get into the old iMAC boot screen then select Windows. It won’t boot in EFI, or using EFI. What did i goof up on? 
What is “an old iMAC”? I have here an
Apple iMac17,1 with retina screen and 2Tb + 128Gb hybrid HDD.
It is loaded with Catalina (MacOS 10.15.7 I think – it is very rarely used) and Leap 15.2. At present it is service as a backup mail server.
It used to always boot into the last used state (normally Leap) unless the option key was used to choose between MacOS (Apple) and the Grub2 menu (EFI). Since the last update it always reverts to prefering MacOS.
When I get time I will upgrade to 15.3 and also tackle the EFI boot behaviour (and arrange an alternative backup mail server).
I had tried Boot Camp to triple boot on a couple of newer iMacs, but gave it up as unreliable with BIOS incompatibilities. I used MacOS to shrink the native partitions, then a bootable USB to install openSUSE in the recovered space.
My old iMac behaved almost like that for a while when I originally installed 15.0, except I never tried running Windows on it. Eventually it seemed the iMac firmware learned it was OK to go right to Grub, and it’s been fine ever since with 15.1 and 15.2 right along with MacOS. Very weird.
Thanks for the quick replay.https://support.apple.com/kb/SP696?locale=en_US is a link to the stats on the model, formerly my wife’s old machine. I dumped the MacOS all together, strictly a linux machine now, or attempting to be once the EFI/GRUB boot issue is solved. Use it primarily for my source code Repos and network monitor
Thanks for the quick reply. " … seemed the iMAC firmware learned …" Could it have been a GRUB update that solved the issue? So when i hold the Option (aka Alt) key the iMAC gives me two options “Windows” or “EFI Boot”
“EFI Boot” causes the iMAC to freeze. Even the mouse is dead, no movement when mouse is moved.
“Windows” causes the iMAC to boot from the built in CD-ROM drive. Which then boots the Leap 15.3 install disk which then boots the Linux distro installed on the hard-drive. If i do not have the CD-ROM drive installed, it will not boot and if i hold the option key it wants to restore via the internet.
I suppose a Grub update could have helped. It also might have had something to do with my use of efibootmgr to define firmware entries. It was a long time ago, memory-wise. There’s a thread or two here from my foibles when originally installing 15.0, two years ago this month IIRC.
What does efibootmgr report when booted to 15.3? Mine:
# efibootmgr
BootCurrent: 0000
Timeout: 3 seconds
BootOrder: 0000,0001
Boot0000* opensuse152
Boot0001* opensuse151
BootFFFF*
# grep GRUB_DISTRIBUTOR /etc/default/grub
GRUB_DISTRIBUTOR=opensuse152
Note absence of a Mac entry, even though MacOS takes up half the 1TB disk. I normally boot MacOS from Grub only. I haven’t needed the Alt key at startup in a very long time.
Again, thanks for the quick replay. 
I get the following “EFI Boot variables not supported on this system.” … This leads me to believe the installation did not correctly install or identify it needed to use EFI … 
… Since this is my 1st EFI booting system i’m at a loss.
That message means 15.3 isn’t installed in UEFI mode. The installation media must be started in UEFI mode to install in UEFI mode. When you booted installation media using the Alt key there probably were two USB choices, and you selected the wrong one.
YEP! That was it … Again thanks for the quick response. 
Since this was my 1st EFI boot machine, lesson learned (learnt and hard, lol). Anyway spelunking around i found a way to move a ‘bad’ old method install to the new method. … Now booting upon restart and cold boot. Thanks again for your assistance. Much appreciated!