dual boot – How to install xubuntu 23.10 in non-UEFI machine

Yes, you can install on an older pre-uEFI machine with all releases of Ubuntu (and flavors like Xubuntu) up to the current Ubuntu 23.10. The system will boot.

The message that worries you is only a warning that it may not boot if your machine has uEFI, but it has been temporarily disabled (via uEFI/BIOS setting). As your machine has no such setting that can be reversed, your system will always boot.

I perform QA-testing using a number of devices, including a

hp dc7700 (c2d-e6320, 5gb, nvidia quadro nvs 290)

which predates uEFI by a number of years; and I get the warning that an ESP (EFI System Partition) isn’t being created (some installer/ISOs will create one anyway; others do not (eg. Lubuntu’s calamares does not create an ESP unless required to boot)).

All releases of Xubuntu up to 23.10 have booted on that pre-uEFI machine I use for install testing.