Thanks so much. Don't laugh, it's a 6th gen CPU in an Optiplex 7050, which is supposed to support Win 7. Most of the technical issues I've been reading about are for more advanced systems that have less native support for Win 7. That's a lot for a newbie to wade through. Of course, it was easy to install Win 7 in MBR. Now I'm considering options for getting over to a UEFI + Secure Boot set-up. Flashboot Pro is not the most convenient, I'm guessing because it changes all the boot files, so they won't secure-boot. UEFISeven might be easier, since I understand it only changes one file, therefore only one hash to flash to the firmware to allow secure-boot. Maybe mbr2gpt.exe could be a good option. I'm only worried about losing the VGA driver when I switch to UEFI. The Flashboot Pro guy developed an INT 10H handler that sounds like it might be a good workaround, but I don't know if I can use it without the full-blown Flashboot repackaging of my boot files. Do I need a third-party tool for the Win 8.1 video drivers? Or can I just copy a few 8.1 files into my installation media? The machine in question has licenses for both 10 & 7. That's why I was planning to copy 7's install.wim into 10's installation USB.
Thanks so much! It's a relief to hear there's a way. Are you secure-booting? Would the procedure for secure-boot be to flash my firmware with the hash from UEFISeven's added file? I would get that hash from certutil?
Since that sevenforums link, just a fewposts back, on the previous page, recommended using the Win 7 install.wim in a Win 10 USB, I wonder if I also need to make monitor.sys available? My theory is the Win 10 boot.wim will be looking for monitor.sys as the pnp vid driver, instead of vga.sys or vgapnp.sys? No .clg file in the Win 10 USB? Will the hardware know that I'm installing Win 7 Ult branding?
UefiSeven solution (UefiSeven, the term already hints Uefi on W7. NOT CSM) simply saying, copy a file bootx64.efi by UefiSeven into two different folders. \efi\boot and \EFI\Microsoft\Boot\, at boot and re-boot. renaming the file at the two folders, once when boot. another before re-boot (2nd boot) That's it. no other changes. no modifications on *.wim (install.wim or boot .wim. typically, for 9th gen - 12th gen, to install W7). If you still want to modify *.wim. a try would be, mount W10 iso, copy its onto a folder, then replace its install.wim with W7 install.wim.
Thanks again, that's probably the approach I'll end up taking. However, before I saw your last message, I did get it working in UEFI+CSM, with the Win 7 install.esd (1.77gb), in lieu of the Win 10 install.esd (3.87gb). However, turning off CSM (the ultimate goal) triggers the VGA driver issue (the issue that UEFISeven fixes). The funny thing is, I could hear Windows 7 boot up without CSM, but the video was frozen. It's also finicky when installed this way. My unsuccessful attempt to install Dell's USB 3 driver also broke my USB 2 ports (therefore, no keyboard or mouse). The Win 10 USB would not recognize my Win 7 system restore points, so I had to go back to Win 7 installation media to run system restore. I'm going to try my 10/7 USB one more time, because I want to see if I can work around the CSM issue described above by installing Dell's graphics driver. Since it booted into 7 without UEFISeven, maybe I can get a functional system with Dell's GPU driver? If it works, then I'll need to find a better USB driver. If I'm able to get all the way to secure boot on my primitive path, without third-party tools, then I can probably live with having to use CSM to access the recovery environment.
Something I've been wondering about UEFISeven. Will it work if I copy the Win 7 install.esd into the Win 10 installation iso? I wasn't sure if that's what you were implying, or if you're saying it hasn't been tried? The only thing holding me back about UEFISeven is that I need to learn key management so I'll be able to secure boot. Sometimes I wonder if it would be easier to secure boot with VMWare or VirtualBox? But I don't know how much risk there may be of an O/S virus affecting my hypervisor.