I am running a 2009 MacPro (5,1) with two separate hard drives: on one, I've installed MacOS High Sierra. On the second, I used Bootcamp to install Windows 10 (legacy/BIOS, not EFI), then installed Ubuntu 18.04 using this guide. I can boot into all three systems using the Apple Startup Manager (holding ALT during bootup). The issue is that when I open High Sierra's "Startup Disk" menu (in System Preferences), I do not see Ubuntu. Only MacOS and Windows/Bootcamp appear in this menu. I'd like to boot into Ubuntu using the "Startup Disk" menu, and I think it is possible if I modify my EFI partitions as described in this second guide.
What would be the right way to do this?
/dev/disk0 (internal, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *2.0 TB disk0
1: EFI EFI 209.7 MB disk0s1
2: Apple_HFS Mac Storage 1.2 TB disk0s2
3: Microsoft Basic Data BOOTCAMP 485.9 GB disk0s3
4: Linux Filesystem 300.1 GB disk0s4
5: Linux Swap 15.0 GB disk0s5
/dev/disk1 (external, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *1.0 TB disk1
1: EFI EFI 209.7 MB disk1s1
2: Apple_HFS Macintosh HD 999.3 GB disk1s2
3: Apple_Boot Recovery HD 650.0 MB disk1s3