make sure neither os x install esd mounted

It could take anywhere from 15 minutes to to 45 minutes to finish, depending on your USB flash drive.

For example, use a live-USB of Boot-Repair-Disk-64bit ( after making sure your BIOS is set up to boot USB in EFI mode.Then just follow the rest of the prompts to start the disk-making process. 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. Last thing I tried just now was the boot-repair tool suggested below the custom GRUB entry in the AskUbuntu post I linked. 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 Try disconnecting any drives that don't contain an operating system.

But selecting it gives me a black screen with the text An operating system wasn't found. 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. I already have secure boot turned off, as you see. There's that 'notebook hard drive' option under 'Legacy boot order', but I can't seem to turn that on. The second image shows what happens if I go into the BIOS boot options. If I don't go into the boot menu and let the computer do its thing, it goes to Windows automatically. 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. I have tried various fixes for both issues: I can miraculously use both, but it's a hassle and all the fixes I've found haven't worked. Now, neither OS recognizes the other's presence. I installed Ubuntu 20.04 alongside a preexisting Win10 installation.

Make sure neither os x install esd mounted