Don't use the Mac-specific installer. Use the normal (server, desktop) installer, install the system using all the defaults (except avoid LVM), and then reboot.
Boot off the USB installer again, but when you see the GRUB menu, press C to bring up the GRUB console.
Finding your installation's drive
At the GRUB console, do
(memdisk) (hd0) (hd0,msdos) (hd1) (hd2) (hd2,gpt3) (hd2,gpt2) (hd2,gpt1)
You may not get exactly the same results as this, but you'll have some options. You want to look for your new installation, not the installer/live system. I suggest finding a partition with your new user's home directory on it:
grub> ls (hd2,gpt2)/home
Keep trying this pattern until you find it.
Finding your installation's boot partition
If you know you installed everything to one partition, you're done — just use the result from the last step, the
(hdX,gptY). If you didn't, or you're unsure, keep the
hdX part, but go through all the
gptY options looking for a
grub> ls (hd2,gpt1)/boot/grub
error: file `/boot/grub' not found.
Nope, not that one.
grub> ls (hd2,gpt2)/boot/grub
unicode.pf2 [...] grub.cfg
Now you want to set this as your root for further commands:
grub> set root=(hd2,gpt2)
Figuring out the boot command
I found that the only way to boot properly was to use the UUID of the drive (ie. no shortcuts with
/dev/sdaX!) - so let's get that:
grub> ls -l (hd2,gpt2)
Partition hd2,gpt2: Filesystem type ext* [...snip...] UUID e86c20b9-83e1-447d-a3be-d1ddaad6c4c6 - Partition start at [...]
Note the UUID string! Yours will be different. Now tell GRUB where Linux is:
grub> linux /boot/vmlinuz[...tab here!...].efi.signed root=UUID=<the UUID from above>
The GRUB console can do tab completion, so if you just type out the
vmlinuz part and hit tab, then hit . and tab again... you won't have to type out the whole filename. But do make sure the
.efi.signed bit is there! And yes, you will have to type out that whole UUID.
Now set the
initrd (initial RAM disk):
grub> initrd /boot/initrd[...tab here!...]
You should find yourself booted into your installation!
I'm no GRUB wizard, so I'd welcome any tips on how to trim this down. Especially the UUID part, that is a little bit heinous.