Check the Secure Boot settings in UEFI.The hard drive is detected in the BIOS/UEFI of your computer.Power and ribbon cables are connected properly.Your hard drive with the OS is connected to your computer (disconnect all other drives and USB devices: this is what you are advised to do in the first screenshot).An HDD with the installed OS is not connected (it may include such problems as poor ribbon cable connection, any hardware issues with the drive, RAID or disk controller).A corrupted boot configuration data (BCD).A damaged system partition, missing or damaged MBR (master boot record), no active partition (or an incorrect disk partition is marked as active).The most common reasons why an OS cannot boot with the “ Operating system not found” error are: Common Reasons of “Operating System not Found” Error