Hi! I pretty much have the same issue(same laptop a515-51G), except it may be worse in my case. The booting from external drive is disabled, and I dont have the password for the bios(my key is 1136186757 in case if somebody wants to help !). I tried removing the CMOS battery that is hidden under the battery. I detached it for about 3 hours and re-attached it. I also tried to set the password flag to 1 in the BIOS firmware settings before flashing it, both methods didn't seem to work. There is a website that gives unlock keys only for 8-digit passwords, They said they should update it to include the 10-digit ones, but, no updates till now sadly. Im pretty much locked. if Windows messes up in some update the laptop is done for.