I did a real mess in my Samsung Ultrabook series 5, formatted the previous boot record in first partition. It was a GPT UEFI installation. I could not repair it, tried thousands of tutorials...
So I decided for a clean install. A simple MBR single partition installation. I removed the HDD and clean it with MiniTool Partition Wizard in another PC. I'd put it back with MBR scheme and only unallocated space. I ran a default Win10 64 bit installation. After all done, I decided to test a "cold" boot (turn off an on). The bios didn't find a bootable device!
Thing is, if I attach the original usb flash (pendrive) installation and don't hit the keyboard to boot from it (when prompted), it will find the HDD windows that initialize normally. I can boot only with the pendrive attached.
My bios is configured to unsafe boot, UEFI and Legacy mode, AHCI auto.
I tried the recovery tools by command line: chkdsk reported ok, bootrec /enum lists the bcd contents but bootrec /rebuildbcd finds ZERO windows installations.