Actually this happened today: I have 3 hard drives on my computer. n 1 hard drive I have Windows 7 and the other hard drive I have Windows 10 installed. But in starting, when I booted in Windows 7, instead of showing C:\, It was showing E:\. Yes. I mean the system drive where Windows 7 is installed. Then my brother by mistake changed the drive letter and drive paths for fun. The Windows 10 drive where windows 10 was installed. The Windows 10 drive showed C:\. He changed it from C:\ to K:\. And I was unaware. When I rebooted the system, It showed an operating system was not found. When I checked the BIOS setup, there was no option for USB Boot even though it was enabled. Please help me. If possible, send me a video tutorial on this. Thank You.