@vicent
probably I have more questions than answers!!!
1) why you want to downgrade to win7; you have retail key and dvd for win 7? (win 8 key may not work on win 7 and win7 downloaded versions may not have the ability to boot in uefi)
2) is the win 8 fully active and licensed?
3) why don't you send the snapshots (or snippings using win 8 snipping tool) of your hdd? this will help us to advise (nay, suggest the better option to) you correctly
4) one reason I have seen in my failures in multiboot install (win7 and win8) is the hidden recovery partition (windows allows only one recovery partition); so normally I use "recovery" folder within the system partition (if you go to the disk management, you see the difference between boot and system partitions);
5) I would suggest that you use a separate partition for installing windows 7; if you send the disk snapshot we can go ahead based on that
regards
neelakantan
(the following links may help you: as installing windows 7 on gpt is not native though some claim that retail dvds have the ability to install in GPT if the bios can successfully answer uefi interrogation by the dvd; that means if you use usb to boot, you need bootmger.efi and tools folder);
Dual Boot Windows 7 with preinstalled Windows 8 on GPT-formatted hard disk and UEFI BIOS.)
"windows cannot be installed on this disk. The selected disk is of - Microsoft Community
Especially the following answers:
Kristoffer Lance
The actual answer is this: It is IMPOSSIBLE to install Windows (7 - 32 or 64 bit, Vista - 32 or 64 bit, etc.) to a GPT partitioned drive, regardless of your BIOS. I have spent 2 straight days trying to accomplish a Windows 7 (64 bit) installation on a 3TB drive, with ASUS UEFI BIOS (June 13, 2012 revision) and NOTHING will allow me to overcome the install screen from the DVD, which will always tell me that Windows cannot install to a GPT drive. I have run Diskpart scripts ad naseum. The only solution to get a Windows install at all is to accept the fact that the drive has to be formatted MBR and take the 1 TB loss. PERIOD. All of the links you will follow in all of the discussions of this situation on all of these so-called MS "tech forums" will waste hours of your time.
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Lachlan0111 OK - I just wasted 2 days on this and FINALLY found solution (for my situation) which is listed nowhere else on the internet (that I can find):
So my mobo requires that the device booted from is prefixed with UEFI: in BIOS. I couldn't make that happen. HDD was prefixed with UEFI: OK.
Eventually discovered that the MSDN Win 7 DVD I was using was the problem; BIOS must interrogate the disc to see if it supports UEFI booting - this MSDN one apparently didn't.
When I tried different (retail) Win 7 media, I could then select my DVD Rom with UEFI: prefix in BIOS.
Voila!
Hope this helps someone else and prevents them from going postal as I almost did.
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regards
neelakantan