I have an Inspiron 3035 laptop, and need to boot from a file on an external drive. (I need to do a repair install.) I can open the UEFL (F2), and I've turned off Secure Boot (I'll turn it back on when I'm done), but I can't figure out how to a) add an external drive, and b) change the boot sequence (which I'll reset when I'm done). Could someone walk me through the procedure?

Thanks.