So today I tried to install Windows 10 as a second OS with a USB. After the installation was complete the setup made a reboot to my laptop and after the Samsung logo it loads to Windows 10 setup again where it began. Windows 10 is installed to C: partition. If I change boot order from USB to HDD first then after Samsung logo I get BOOTMGR is missing. Please help as I don't want to lose my files from C: (I was dumb and didn't made a backup)