A note which might help:
Sometimes when booting from an external drive such as a USB it helps to, upon startup, press the key combination to enter BIOS/UEFI anyway, then just exit the UEFI/BIOS settings without saving changes. I don't know why this happens, it does with my main PC, but some PCs, even when you've already set up and saved the preference in the BIOS/UEFI for the USB ports to be the first things they check for booting from, only seem to boot from external drives (if an existing hard-drive is fitted) when you go through BIOS/UEFI during startup like this.
Another tip to ensure booting from an external drive could be to carefully and temporarily take out the internal hard-drive, MrsRobot have you ever opened up your PC before and would you be familiar with how to remove your HDD for a while if other methods fail to let you boot from USB?
Edited by rp88, 18 May 2018 - 05:44 PM.
Back on this site, for a while anyway, been so busy the last year.
My systems:2 laptops, intel i3 processors, windows 8.1 installed on the hard-drive and linux mint 17.3 MATE installed to USB