This might have happened because of buggy firmware, bugs in software, deliberate action on the part of a user, an accident by a user, or for other reasons. The OSes added their entries, but they were subsequently deleted.If you've installed OSes and they do not show up in this way, there are basically two possible explanations: Thus, if you have other (EFI-mode) OSes to boot, they should already be noted in the boot manager list. Ordinarily, when an OS installs itself on an EFI-based computer, it copies its boot loader to the EFI System Partition (ESP) and adds an entry to the firmware's boot manager list, which is stored in NVRAM. BIOSes don't have boot manager menus like the one you describe at best, with a BIOS, you'll see a list of boot devices ('Disk X,' 'CD-ROM drive,' or whatnot), not a description like 'Windows Boot Manager.'.

Thus, if you really installed to a GPT disk, your computer must have an EFI. Windows refuses to install to a GPT disk when it's booted in BIOS mode.I'm quite certain you've got an EFI and not a BIOS for two reasons: It's not an EFI is a replacement for the decades-old BIOS, and is radically different in design and capabilities - although both EFIs and BIOSes do handle the task of starting the computer's boot process. Unfortunately, many people, and even many manufacturers, refer to their EFIs as 'BIOSes,' but that creates confusion because people think that an EFI is just an updated sort of BIOS or a new BIOS feature. You don't have a BIOS you have an EFI (or UEFI, which is EFI 2.x), which is a replacement for a BIOS.

Uefi windows boot manager