Start tapping the Esc key BEFORE starting the PC and then press the start button.
Keep tapping the Esc key until a popup menu shows. Then pick Set Up from that popup menu, F9.
If the USB is bootable, it will show in the menu. Make the USB the first item and then save.
It should then boot from the Linux Mint Live. BTW: you are doing the best way I would recommend, when asked – just make sure there is media attached IE: external hard drive or BIG capacity USB drive to hold the files that are being saved from the failed hard drive.
Post back if additional help is needed.