The hardware in point is an oldish desktop with a msdos partition table and booting in BIOS mode. The objective is to change the operating system arrangement from
- dual boot Ubuntu 14 + Windows
to
- Ubuntu 16.04 only
by using a bootable USB pen drive with a Live 16.04 (gpt partition table) and by redrawing the partition table with the install option "Something else".
Early in the installation procedure I receive the following message
The machine's firmware has started the installer in UEFI mode but it looks like there may be operating systems already installed using "BIOS compatibility mode".
If you continue to install Debian [sic] in UEFI mode, it might be difficult to reboot the machine into any BIOS-mode operating system later.
If you wish to install in UEFI mode and don't care about keeping the ability to boot one of the existing operating systems, you have the option to force that here.
If you wish to keep the option to boot an existing operating system, you should choose NOT to force UEFI installation here
Go back/ Continue in UEFI mode
I am slightly confused by the diagnose above because the machine has no efi directory inside /sys/firmware to start with, so the first diagnose should not apply. Also, any operating system should be in BIOS mode in this machine.
Not knowing the inner workings of this request, I would go for 'go back', which in passing means 'go forward along the other way'. However, I am unsure about what really applies to my case, and better safe than sorry.
Would an expert/experienced fellow explain which option is safe?
Not so old as to be BIOS only.
How you boot install media, UEFI or BIOS is then how it will install. And grub will not correctly install unless you have an ESP - efi system partition if UEFI. You need to reboot flash drive in Legacy/BIOS/CSM boot mode.
You should have two options in UEFI, if UEFI Secure Boot is off. But some tools to create a flash drive may now only create a UEFI version. ISO is configured for both BIOS & UEFI, so also check how you made installer.
Do not know nor currently use encryption. FileSystem encryption https://www.kernel.org/doc/html/latest/filesystems/fscrypt.html ecryptfs was dropped from the Ubuntu installer and deprecated in 18.04 LTS in favor of full disk or manually using fscrypt Encryption discussion /home phasing out, use fscrypt for per directory encryption https://bugs.launchpad.net/ubuntu/+source/ecryptfs-utils/+bug/1756840
And I do prefer even in BIOS mode to use gpt partitioning, but then you need bios_grub partition. But that requires either conversion which may or may not work or total redo of hard drive.