I have an Inspiron 7591. I wanted to boot off a USB Stick. However, it wouldn't show the boot list option for the USB. Even the BIOS doesn't recognize it. It boots fine on Inspiron 3521. I turned off secure boot, and PTT(whatever that is). I searched the internet and it said to boot from a USB 2 port. But there are only USB 3 ports in the laptop. How do I fix this ? Thanks in advance.