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I just accidentally set the system partition C:\ as active in Windows 7's disk management, trying to get this absolutely useless Windows backup & restore program to work. Now I can no longer boot: Bootmgr is missing. Press Ctrl + Alt + Delete to restart. How can I set the 100 MB boot partition in front of it back to active?

Using Linux Mint's disk image writer, I already created a bootable Windows 10 USB flash drive, but the system just won't recognize it - what's the matter? I played around with the BIOS settings (boot order, USB support, some CMS/UEFI/legacy boot and Secure Boot settings etc.), but it just won't start from the USB flash drive.

What's even more strange: I'm getting a CPU Overtemperature Error! Press F1 to Run SETUP at every single reboot and the CPU temperature is hitting 84-86 °C even though I did not (consciously) change any setting related to the CPU. What have I done? How can setting the wrong partition as active have such unforeseen consequences? The BIOS offers advanced controls for CPU overclocking etc. - is it possible that BIOS saves some of these settings in the boot partition and now it's unable to load them?

Please help...I need to get this fixed fast. It's a workplace PC required for measurements and I was just in the process of creating the first system disk image when this mess happened (got no recovery options). The PC has no CD/DVD drive, so I'm stuck with using USB flash drives to get this solved.

PS.: I will post detailed information on the ASUS BIOS shortly.

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    Things you can try : re-creating windows 7 bootable USB (hope you have the iso) from linux(mint in your case)(instructions - ghacks.net/2018/05/25/windows-bootable-usb-woeusb/amp) or just partition drive and install linux mint (can remove afterwards if not needed) and use linux mint grub to boot windows or use boot repair (it comes pre-installed in newer linux mint live-cd to repair and restore windows bootloader or try setting boot flag from linux (unix.stackexchange.com/q/343429/314322)
    – Madhubala
    Commented Aug 8, 2020 at 23:43
  • Okay, I'll try it first with a live Linux Mint USB flash drive (hopefully it will boot) and try to set the boot flag with fdisk
    – david
    Commented Aug 8, 2020 at 23:55
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    For CPU Overtemperature Error - maybe you need to replace the thermal paste or make sure the fan attached to your cpu cooler is fully plugged in and working properly ; and if you think you have messed up the bios setting then there is an option in bios something like load optimised defaults and load failsafe defaults , choose any one of them
    – Madhubala
    Commented Aug 9, 2020 at 0:02
  • Thanks, I saw the option for default settings, but didn't want to touch the BIOS yet, as I didn't set up the system (don't want to break anything). The BIOS options are quite extensive, I will upload screenshots shortly... Regarding the CPU overtemperature issue, I'm positive this is directly related to the boot problems as there has never been anything temperature-related in the past. Also, the PC is not incredibly old and the i7 has a huge heatsink-cooler + fan on it
    – david
    Commented Aug 9, 2020 at 0:09
  • is it possible that BIOS saves some of these settings in the boot partition and now it's unable to load them? - No
    – Madhubala
    Commented Aug 9, 2020 at 0:14

2 Answers 2

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Things you can try : re-creating windows 7 bootable USB (hope you have the iso) from linux(mint in your case)(instructions - https://www.ghacks.net/2018/05/25/windows-bootable-usb-woeusb/amp/) or just partition drive and install linux mint (can remove afterwards if not needed) and use linux mint grub to boot windows or use boot repair (it comes pre-installed in newer linux mint live-cd to repair and restore windows bootloader or try setting boot flag from linux (https://unix.stackexchange.com/q/343429/314322)

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Expanding a bit on Madhubala's answer, this is the step-by-step solution that worked for me:

  • 1.) Create a live (= bootable) Linux flash drive from a downloaded ISO. This is super simple, from Linux using gnome-disks (menu > Restore Disk Image > follow instructions) or from Windows using e.g. Rufus. For some reason, creating a live Windows flash drive never works for me, so I stick with Linux.
  • 2.) Launch Linux from the flash drive, open a terminal, and reset the boot flag to the correct partition using fdisk as described in this answer.
  • 3.) Now, Windows should boot normally! The accidental "active" (boot) flag can be removed using diskpart from the Windows command line as described in this answer.
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