First, your language is confusing: "EFI check" is not a standard term. To help facilitate communication, here are some definitions (my apologies if this seems pedantic; but it's necessary for clarity):
- BIOS -- The firmware used on most PCs sold between 1983 and 2010. Often used (confusingly) to refer to more recent firmware. Importantly, older boot loaders for DOS, Windows, Linux, and other OSes were written with BIOS in mind.
- EFI or UEFI -- The firmware used on most PCs sold since mid-2011. Among other things, EFI requires a new type of boot loader.
- CSM -- The Compatibility Support Module, which enables an EFI to boot a BIOS-mode boot loader, in much the way that WINE enables Linux to run Windows programs.
- Legacy -- In this context, a synonym for CSM.
I suspect that by "EFI check" you mean that the computer is favoring the EFI-mode boot loader over the BIOS-mode boot loader. Note that the OS's boot loader doesn't check for the boot mode, as you seem to believe; the firmware checks for boot loaders written for the mode(s) it supports, and selects one based on whatever algorithm it uses and whatever options you as the user have set or select at boot time. It's the firmware that determines the boot mode. The Ubuntu installer can influence this only by presenting BIOS, EFI, or both boot loaders and letting the firmware decide which to use.
Be aware that EFI-mode booting does offer some advantages over BIOS-mode booting, so you may want to consider using it for both Windows and Linux. Most obviously in the short term, EFI-mode booting is often slightly faster than BIOS-mode booting, so you can shave a few seconds off your boot times by using EFI. EFI-mode boot loaders are also stored as files on the disk rather than as binary blobs written to officially-unallocated disk areas (as BIOS requires), so managing EFI boot loaders is theoretically easier. (In practice, some systems have EFI bugs that negate this advantage.)
That said, if you truly want to install your OSes in legacy mode, you can force a legacy-mode boot; however, the details of how to do so vary from one computer and OS to another. Some things you can try include:
- Enable BIOS/CSM/legacy support in your firmware setup utility. This is likely to be a requirement, but the details vary greatly from one computer to another. The option name may be non-obvious, and the details of what the option offers vary from one system to another. Thus, I can't be very precise about this; you'll just have to dig into your firmware and experiment.
- Disable EFI support in your firmware setup utility. This is possible in some cases, but usually it's not possible on true EFI-based systems. VirtualBox enables you to completely disable EFI support, but on most PCs, options that users read as disabling EFI-mode booting usually just favor legacy-mode booting rather than completely removing EFI-mode support. Thus, disabling EFI support may be a theoretical possibility, but it's unlikely to be a practical option for you.
- Use your firmware's built-in boot manager to force a BIOS/CSM/legacy boot of your boot medium. The details of how to access the boot manager vary between computers, but it's usually done by hitting a function key (or sometimes Esc) early in the boot process. You may then see a boot menu, and with any luck it will have two options for your boot medium. One will include the string "EFI" or "UEFI" and the other won't. Select the option that lacks the EFI/UEFI string and it should boot in BIOS/CSM/legacy mode.
- Use rEFInd to force a BIOS-mode boot. Download and prepare the USB flash drive image of rEFInd, then mount it and edit the
EFI/BOOT/refind.conf file on the disk. Uncomment the
scanfor line, remove the
external items, and add the
biosexternal items. This will cause rEFInd to omit EFI-boot options for external media and add BIOS-boot options for same. Then insert both rEFInd the USB flash drive and your Ubuntu disc and reboot into rEFInd. It should present a boot menu, from which you can select your Ubuntu (which might conceivably be mis-identified or poorly labeled). Note that this will work more reliable with an Ubuntu CD than with an Ubuntu USB flash drive. If you use the latter and it doesn't work, try swapping the two USB flash drives.
- Hack the Ubuntu image to remove the EFI boot loader. This is most easily done from a USB flash drive, so prepare an Ubuntu installer on USB flash drive. You can then remove or rename the
EFI directory tree on the USB flash drive and it should stop booting in EFI mode.
One more suggestion:
- At one time there was a special version of Ubuntu for Macs. Ironically, this version lacked EFI support. The idea was to install it in BIOS mode, since EFI-mode installations of Ubuntu on Macs often had problems with hardware initialization. In any event, if this variant is still available, you could try using it.