SOLVED: I mis-installed it. Thanks David.
I installed Ubuntu 20.04 alongside a preexisting Win10 installation. Now, neither OS recognizes the other's presence. I can miraculously use both, but it's a hassle and all the fixes I've found haven't worked. I have tried various fixes for both issues:
When I boot and go into startup menu > boot device options, I'm faced with this (first image) the bottom option "Notebook hard drive" starts GRUB and I can get into Ubuntu normally. But why isn't it recognized as Ubuntu? If I don't go into the boot menu and let the computer do its thing, it goes to Windows automatically. The second image shows what happens if I go into the BIOS boot options. There's that "notebook hard drive" option under "Legacy boot order", but I can't seem to turn that on. I already have secure boot turned off, as you see.
Getting the computer to start directly into GRUB would presently be disastrous anyway, because GRUB in turn doesn't recognize Windows and I'd be unable to use it! I've tried this (second answer, "If the os-prober method doesn't work...") and indeed, now there's a Windows 10 entry. But selecting it gives me a black screen with the text An operating system wasn't found. Try disconnecting any drives that don't contain an operating system. Press any key to restart
Note that I don't actually have any rows in my lsblk output with /boot/efi in them, but /dev/sda2 has an appropriate label:
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
loop0 7:0 0 55,5M 1 loop /snap/core18/1988
loop1 7:1 0 55,5M 1 loop /snap/core18/1997
loop2 7:2 0 219M 1 loop /snap/gnome-3-34-1804/66
loop3 7:3 0 64,8M 1 loop /snap/gtk-common-themes/1514
loop4 7:4 0 51M 1 loop /snap/snap-store/518
loop5 7:5 0 31,1M 1 loop /snap/snapd/11036
loop6 7:6 0 32,3M 1 loop /snap/snapd/11588
sda 8:0 0 447,1G 0 disk
├─sda1 8:1 0 499M 0 part
├─sda2 8:2 0 99M 0 part
├─sda3 8:3 0 16M 0 part
├─sda4 8:4 0 186,3G 0 part /home/kilian/Windows
├─sda5 8:5 0 603M 0 part
├─sda6 8:6 0 186,3G 0 part /
└─sda7 8:7 0 15M 0 part
sr0 11:0 1 1024M 0 rom
/dev/sda2: UUID="C495-5A59" TYPE="vfat" PARTLABEL="EFI system partition"PARTUUID="6795db25-5a95-4724-a256-4b7cf3c60931"
Last thing I tried just now was the boot-repair tool suggested below the custom GRUB entry in the AskUbuntu post I linked. I got this:
WindowsEFI detected. Please disable BIOS-compatibility/CSM/Legacy mode in your UEFI firmware, and use this software from a live-CD (or live-USB) that is compatible with UEFI booting mode. For example, use a live-USB of Boot-Repair-Disk-64bit (www.sourceforge.net/p/boot-repair-cd), after making sure your BIOS is set up to boot USB in EFI mode.
Anyway, before I continue trying out fixes that work on either problem in isolation - is this even two separate problems? Or are both linked to the same issue, did I do something fundamentally wrong when partitioning my SSD? Has anyone encountered this particular problem?
