Being in desperate need for finally getting the recent LM version (since LM 17 no longer meets certain Internet requirements), I finally made a second attempt to install it as an extra OS on an extra drive (sdb). And, while the first attempt a month or two ago had looked like it was working but at the end had utterly failed (possibly because legacy mode was allowed), this time -- following good advice -- I did get LM 20.1 MATE installed,
BUT I was stabbed in the back nevertheless.
I followed the
viewtopic.php?p=2023951#p2023951
suggestions as much as possible. (sadly NOT 100% possible)
After setting "UEFI/Legacy Boot" to [UEFI Only] and "CSM Support" to [No] in my "ThinkPad Setup" (which is the name of the UEFI-firmware BIOS-like interface on my main laptop reached with the F1-key during startup), my LM20.1 MATE installer DVD would no longer boot (many, many tries). I even burnt me a new LM20.1 MATE installer DVD after having booted LM 17 on sda under these conditions in the hope to get a LM20.1 MATE installer DVD that would boot under these UEFI-only settings. But it didn't help.
So, I switched "CSM Support" on again (but left the "UEFI/Legacy Boot" at [UEFI Only]). I still had several boot failures, but eventually the DVD finally did boot. (Who knows why?)
I then went through the many (and partly very obscure) steps of installing LM 20.1 MATE on sdb2 with its ESP on sdb1 (sdb1 to be exact, but asked by me merely for sdb as suggested). (And, by the way, in the partition table making with GParted I had to manually switch from a silly [ms dos] default setting to [GPT], though. Yikes! MSDOS as default in 2021???)
Picking sdb2 for the LM 20.1 MATE install was incredibly obscure to achieve. Clicking on the partition for "Change" only gave drop-down options not including this. When I finally picked the Ext4 blabla from it ("Ext 4 journaling file system") I finally got another dialog where I could enter a "Mount point" where I then entered the root slash (/) and -- because perhaps not yet formatted by GParted -- I also checked the formatting checkbox. That finally worked.
But...
The backstab:
When restarting, LM 20.1 MATE booted by default WITHOUT giving me a GRUB2 menu, AND when I entered the "ThinkPad Setup" (the UEFI firmware BIOS equivalent) I only found among booting options the Windows 8 (that's on sda like LM 17 is) and Ubuntu (set at the top of the boot order). This Ubuntu clearly is LM 20.1 MATE. But the LM 17 (which I think was also called Ubuntu in the BIOS equivalent (but I don't know for sure) while maybe actually called LM 17 in the GRUB2 menu (also not sure)) has disappeared.
Apparently the LM 20.1 MATE installer erased the sda's LM 17 "Ubuntu" entry from the firmware's NVRAM. Thanks a lot!
--- And this despite me using the 'something else' option! I wonder if the switching off of the esp and boot flags on the sda's ESP partition or the 'something else' option itself is the reason for the installer (or the firmware ?) to erase the access to the older Linux Mint installation. (Or could it have even been my error of forgetting to re-establish the esp and boot flags on the sda's ESP partition before the restart (an error I fixed after the restart)?)
Out of my utter despair (needing the well configured LM 17 to continue my daily things to do until in the far future I have LM 20.1 properly configured) I then installed rEFInd even though on my other laptop ("travel laptop") rEFInd had been hit with "secure boot" blue screens DESPITE having "secure boot" turned off. After all, I had figured out how to let the other laptop boot again (by moving rEFInd in the UEFI firmware (BIOS equivalent) down from the top of the boot options). So, I were fairly sure I could do that on my main laptop, too, if necessary.
BUT -- Surpise! -- on this laptop rEFInd has not been attacked with those blue screens. So, GRUB has utterly abandoned me (even update-grub and boot-repair did nothing for me) and only rEFInd rescued me. Let's hope rEFInd stays alive for years to come. And tips of how to fix the GRUB and/or UEFI firmware mess if it's possible would be welcome.
Holy moly!