Earlier when my laptop was in legacy mode it readily booted Kali Linux via USB, creating it through UUI.Recently,I switched to UEFI but it's not booting from USB anymore. In the boot device option there is no mention of my USB (PENDRIVE NOT SHOWING) and boot priority order is also given to USB disk set key but nothing is helping and yes I want to dual boot it with my windows 8.1 pro. Someone help please.
There will be no clear solution, until you establish how Windows 8 Pro was installed originally.
Check if UEFI or Legacy in WinPE for Windows 8 and 8.1
NOTE: This option can be useful if you are unable to boot into Windows or in WinPE.
Open a command prompt at boot.
In the command prompt at boot, type the command below and press Enter.
- In the command prompt at boot, type the command below and press Enter.
reg query HKLM\System\CurrentControlSet\Control /v PEFirmwareType
This command returns 0x1 if the PC boots in Legacy BIOS mode, or 0x2 if the PC boots in UEFI mode.
Other options check this site: http://www.eightforums.com/tutorials/29504-bios-mode-see-if-windows-boot-uefi-legacy-mode.html
Once you have that answer you can correctly set your BIOS, to avoid any future conflicts before installing another OS alongside Windows.
Pen drives and optical DVD discs show up as UEFI (when booting in UEFI mode) on my newer computers, though my Lenovo T420 laptop refuses to boot in UEFI, but works just dandy in Legacy mode.
All the best hopes this help you :)