First of all, the bootable USB will not show in the GRUB menu, but it will just boot before GRUB loads. Something is wrong with the bootable USB. How did you create it?
I would use dd. It's a command line based utility that, among other things, can create a bootable USB stick.
So download the Mint image to, say, your Downloads folder, insert the USB stick and open a terminal.
The steps are as follow:
$ cd Downloads
$ lsblk
The output shall be something like this:
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 465,8G 0 disk
├─sda1 8:1 0 100M 0 part /boot
├─sda2 8:2 0 42G 0 part /
├─sda3 8:3 0 1K 0 part
├─sda4 8:4 0 242,1G 0 part
├─sda5 8:5 0 48,8G 0 part /home
├─sda6 8:6 0 8G 0 part [SWAP]
└─sda7 8:7 0 124,7G 0 part /media/data
sr0 11:0 1 1024M 0 rom
sdb 8:16 0 465,7G 0 disk
└─sdb1 8:17 0 465,7G 0 part
Try to spot out which the USB drive is. You will easily recognize it based on the SIZE column. Please note that you need to spot out the USB stick and not any of its partitions (that is, the ones with TYPE = part). If any of the partitions of the USB stick are mounted, first unmount them all by typing for each of them:
$ sudo umount MOUNTPOINT
Then write the iso image to the USB stick (ALL THE DATA IN THE USB STICK WILL BE DESTROYED):
sudo dd if=<mint_iso_image> of=/dev/sdX
where X is the letter that corresponds to the USB stick and should definitely not be "a".
Disclaimer: Please note that if you are not sure which is your USB stick (or pick a wrong one), this process may have disastrous effects (ie. destroy your data on your hard disk). Please proceed with caution!
Regards, Errikos.
I'm currently using Ubuntu 13.04
" ... in 2109, six years later ?!