Lenovo G50-30 Dual Boot Windows 7, 8.x - IMPOSSIBLE!

What is BIOS BACK FLASH? Is that a setting in the BIOS or a description you're making of needing to downgrade BIOS before WIn7 will install?

option BIOS BACK FLASH is in lenovo bios, you must enable this options (disabled by default) because bios flash tool with older bios version will not be able to downgrade bios from 43ww to 40ww version. to install win7 on lenovo g50-30 you must have bios in a7cn40ww version if you have 43ww version, win7 installation will not pass (BSOD 0x000000a5, bios is not acpi compilant etc...)

Background information:
I want to dual boot Windows 8.1 and 7.
System is pre-installed with windows 8.1
System is on a GPT drive
I made a Drive partition created for windows 7, and I want to install windows 7 to a GPT partition, not MBR.
UEFI is enabled, fast boot disabled, secure boot disabled.
I have also formatted a USB stick with win 7 for GPT(UEFI).

The problem:
I'm trying to install to the GPT partition I created for windows 7. However, I am unable to boot from USB.
UEFI does not allow me to boot from USB.
If I enable legacy bios, USB is detected but cannot be booted as the USB is formatted for GPT.
If I use UEFI mode, I can only boot into windows 8.1 and I cannot boot from USB.
How can I boot from the Windows 7 installation USB? Please help.

Does WIndows 8.1 boot with Secure Boot disabled?

Did you format the flash installer using only Option One of UEFI Bootable USB Flash Drive - Create in Windows?

Did you revert to the previous BIOS as suggested in Post 12 to be the only way to install to the G50-30?

Yes, windows 8.1 boots with secure boot disabled. I did format the USB using option 1 successfully.(GPT option for UEFI) No errors were present in the log.

Edit: Post 12 may be incorrect. You do not have to revert the bios to install windows 7.
To solve the ACPI non compliant problem: In the BIOS
1.exit tab>change windows8OS optimised defaults to win 7 optimised defaults>
2.load default bios values>
3.change legacy mode to UEFI>save and exit.
Edit 2: I do not know if the above applies to the newer BIOS, my BIOS version is 40ww.

I have now installed win 7 to a GPT partition without using the USB, since it was unable to boot from the USB

The issue has not been completely solved however. After the win 7 installation, it automatically attempted to reboot into win 7, so I had to manually stop it from booting into win 7, and change the bios settings from 'win864bitOS' to win7, load setting defaults, change legacy to UEFI, save and exit. If I did not change the bios, windows 7 would not even attempt to boot and the installation would fail, thus causing it to automatically revert the installation.

So now even though win 7 is installed, it cannot boot if I load default bios settings for windows 8 and other OS. A red bar will flash at the top if I try to do this. If I change the bios to win 7 OS with UEFI enabled, it can boot, but windows cannot start. The windows 7 logo appears and shines brightly, but that is as far as I can get. I am unable to start windows 7. I believe that the default bios settings for win 7 OS is only compatible with MBR installations and not GPT, even if I enable UEFI. Please advise.

I was never able to get my Lenovo G500 to Dual Boot WIndows 8 which it came with and Windows 7. I had to delete all partitions to get it installed to UEFI. One thing I later learned that I did not try is to choose Other OS or Windows 7 OS defaults and then Exit saving those settings. Perhaps that will allow the Dual Boot, but I doubt it.

There are many BIOS' which will not allow keeping the factory Windows 8 install with a new Win7 UEFI install in Dual Boot, until the drive is wiped or all partitions deleted Win7 will not install. Then you can even install Windows 8 to a second partition and achieve a Dual Boot with UEFI, but it will not work with the existing one except on certain rare BIOS'.

I think what you said is true, Lenovo G50 refuses to boot from the win 7 installation USB, so I did not use the USB. Choosing win 7 OS defaults(using the method I mentioned in my previous post) will allow win 7 to install in UEFI mode successfully(while having win 8 pre-installed), but win 7 will not start. It will be successfully installed because win 7 will boot and get the win 7 logo, but it cannot go past that point. I suspect this is due to Lenovo's sub-standard BIOS.

Can we see a screenshot of Disk Management - Post a Screen Capture Image?

Have you run a Startup Repair on the Win7 install that will not start?

Have you tried installing it again?

Where did you get the Win7 ISO? Confirm it using Step 1 of Clean Reinstall Windows 7

Not yet, I'll try the startup repair after this post.
No, I haven't tried installing it again, because I will need to sit through the whole installation in order to prevent it from rebooting into Win 7 during the installation. This, currently, is a clean install.
The ISO was previously obtained from Digital River.