my pc is an ASUS RoG g750jx with dual boot : Debian Jessie and windows 8.1 , until yesterday everything was fine. Yesterday booting my pc it didn't display GRUB and directly booted to Windows,i thought : "not bad i just have to reinstall GRUB". I wanted to boot from usb (it works on others computers) using a Debian-live ,as soon as pc boot i press ESC key to get the boot override menu' , from the list i select the usb device after few istants it send me to BIOS. it happens with every usb i tried (i tried all the ports). Until yesterday i was able to boot from usb without problems ,since my GRUB is magically disappeared i can't boot no more . Seems like some sort of error during usb boot that sends me to BIOS.
Bios settings :
secure boot : disabled Legacy : disabled Usb support : enabled
I didn't change any settings and before yester with the same configuration i was able to boot from USB device!