OK, I think I'm confusing you with the BIOS boot priority order and the F12 boot manager menu that pops up when you enable it and press F12 after switching on. In BIOS under the Boot priority menu, I see all the devices - USB HDD, USB CDROM, USB FDD, the SSD disk (it is listed as EMMC: HB4Ge 32G) and Network boot devices. It is true that I see them all there in BIOS whether I am in UEFI or Legacy mode and can order and list them appropriately. The main difference is that when I switch from UEFI to legacy, Windows Boot Manager device disappears. However, when I enable the F12 boot menu on the device, if I am in UEFI mode, I only see the SSD and any UEFI imaged devices. When I switch the legacy mode, again I see all the devices in the BIOS boot priority list, and when I boot and press F12, I see any legacy imaged devices appear in the boot selection list. I also see the SSD, but it will not boot from it in legacy mode.
Eg. of the BIOS Boot Menu screen when set to UEFI:
Boot Mode: UEFI
Secure Boot: Disabled
Boot priority order:
1. USB HDD:
2. USB CDROM:
3. Windows Boot Manager
4. EMMC: HB4Ge 32G
5. USB FDD:
6. Network Boot-IPV4:
7. Network Boot-IPV6
So when I am in UEFI mode, and I have an official Win 7 install CD in the USB CD, it simply will not boot off the CD.
Whenever I switch from UEFI to Legacy it greys out the boot priority list and pops up a message saying the boot devices will not be updated until reboot.
And this is what it looks like in legacy mode:
Boot Mode: Legacy
Boot priority order:
1. USB HDD:
2. USB CDROM: HL DT STDVDRAM GP60NB50
3. EMMC: HB4Ge 32G
4. USB FDD:
5. Network Boot: Realtek PXE B01 D00
It will boot from the CDROM in this mode, but it will not boot from the SSD. Note that it has actually detected and listed the device connected as the USB CDROM (GP60NB50 is the model number of the drive). This is one of the key ways I know the device is going to boot in particular mode - you see a description.
Hope that explains. This machine seems to only boot legacy (MBR) devices in legacy mode and UEFI (GPT) device in UEFI mode. Hence my desire to change the SSD from GPT to MBR.