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If you're unable to select the Repair Windows boot files option because it's grayed out, refer to this answer. The partition on which Ubuntu is installed can be identified from the Disks application which is built-in in Ubuntu. The boot flag should be placed on the same partition on which Ubuntu is installed. Open the Boot Repair application and select Advanced Options -> Other Options tab -> Repair Windows boot files.
#Active partition not found press any key how to
![active partition not found press any key active partition not found press any key](http://woshub.com/wp-content/uploads/2020/06/operating-system-not-found-erro-on-windows-boot.png)
Sometimes the manufacturer's splash screen is a part of the Windows bootloader, so when you power up the machine it goes straight to the GNU GRUB menu screen, and then pressing Shift is unnecessary. With UEFI press (perhaps several times) the Esc key to get to the GNU GRUB menu screen. If this doesn't work when the GRUB menu does not appear at boot time, immediately after the BIOS/UEFI splash screen during boot, with BIOS, quickly press and hold the Shift key, which will bring up a GNU GRUB menu screen. If there's not one, or if the screen goes by too fast to see it, check your manufacturer's website. During startup, there's often a screen that mentions the key. On tablets, common buttons are Volume up or Volume down. Common keys used are Esc, Delete, F1, F2, F10, F11, or F12.
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If the computer boots into Ubuntu automatically without showing the GRUB menu at all, sometimes you can press the manufacturer's BIOS/UEFI key at boot time and while in UEFI or BIOS mode from the firmware menus select the operating system you need.