You can easily install Ubuntu 14.04 LTS, Linux Mint 17 from USB.
How to create bootable usb for Ubuntu 14.04 LTS, Linux Mint 17 on Windows or OS X with unetbootin.
You can install unetbootin easily For WIndows and OS X, you can download unetbootin here.
After You install unetbootin. Now open your unetbootin Application
Browse Ubuntu or Linux mint iso file (1) Which you would have already downloaded before starting this, right ;)
Browse usb disk (2)
And finally click OK and wait until finish, and then restart
(Dont forget to set first boot on your bios to USB)
Note (USB format must FAT 32)