Windows – Can only install Windows 7 in UEFI-Legacy Mode

It's possible, and it's very frequently done with both external USB sticks and internal drives.

Regarding partition table types:

• BIOS normally doesn't need any partition table. It is only interested in the bootstrap code part that is the first 440 bytes of your MBR. (Although there are exceptions. Some BIOS implementations actually do break if they cannot find a MBR with one of the usual partition types. Hopefully those are rare.)

  The GPT partition table doesn't physically replace a MBR – it always starts at sector 1, while the MBR is in sector 0 – so it's possible for a disk to have both. In fact, most GPT disks do have a "protective MBR" containing just a single partition that covers the entire disk, to prevent older MBR-only partitioning tools from accidentally destroying the data.

  So you can install, for example, the Syslinux boot sector into your MBR, and it will boot. (The syslinux-install_update script will do this for you, but you can look for gptmbr.bin in your syslinux package to do it manually.)

  Much like the "active" flag on MBR partitions, Syslinux will look for the "legacy BIOS bootable" flag on GPT partitions (bit 2 – UEFI spec 2.4 section 5.3.3 table 20); the aforementioned script also sets that flag automatically if you have sgdisk (from gptfdisk) installed.

  GRUB can also be used, but it's also a bit more annoying. Since GRUB 2 wants to "embed" parts of itself in the traditionally-unused sectors 1-62, it would overwrite a GPT that's stored in the same location. So you might need to create a dedicated partition of 2-4 MB and mark it as BIOS-bootable, so that grub-install would find itself an embedding place.

• On the other hand, the UEFI specification does require support for MBR partition types (section 12.3.1) and assign the MBR partition type 0xEF to the EFI system partition (section 5.2.2).

  So if you for some reason cannot get your disk working with GPT, you still can create an EFI system partition on MBR, and it will be used as long as it has the correct filesystem and all the necessary files (section 12.3.1.3):

  For removable media devices there must be only one UEFI-compliant system partition, and that partition must contain an UEFI-defined directory in the root directory. The directory will be named EFI. All OS loaders and applications will be stored in a subdirectory below EFI called BOOT. There must only be one executable EFI image for each supported processor architecture in the BOOT directory. For removable media to be bootable under EFI, it must be built in accordance with the rules laid out in Section 3.4.1.1.

Regarding operating systems, Linux generally does not care the slightest bit about whether it's booted from BIOS today and from UEFI tomorrow. I've had to reboot my laptop from UEFI mode to "BIOS compatibility" mode and back maybe a hundred times this week, while trying to find a bug in the 3.17 UEFI support...

Note that the "BIOS to UEFI Transformation" article is about a quite different thing. It describes how to install UEFI itself – an UEFI implementation called "DUET" – to be started from a BIOS-only system. It has nothing to do with booting an operating system directly from BIOS.

Chances are you were booting in just one mode all along. One of the problems with the built-in EFI boot managers is that they don't always do what you think. When you select Option A in the boot manager, it might try that option, but then if that option fails, it might start going through its regular boot order, so you might end up booting via Option B or Option C even though you selected Option A. (I've been frustrated by this tendency myself on more than one occasion. I keep screaming at the computer to do it what I tell it to do, but it never listens. ;-) ) Thus, your attempt to boot in two modes might have ended up booting in just one mode.

AFAIK, it's not possible to install Windows in a way that will boot a single installation in either BIOS/CSM/legacy mode or in EFI/UEFI mode simply by selecting a different boot option. If it is possible, and if that really is what you've accomplished, chances are nobody else (except possibly some Microsoft developers) could tell you how to do it.