I wrote a Linux Mint 18.1 image using the Linux Mint USB Image Writer utility and everything went fine. However, my original usb had a capacity of 64 Gb and now only shows 1.8 Gb (the size of the image). I know I can format it and recover again the whole capacity but I wanted to know if there is any utility to write a working image and still access the whole device in order to store data.
What you need is a partition editor to create a partition in the unused space of your USB drive. Then you can create a filesystem in that unused space. I'm not very familiar with it but I believe
gparted will let you do both through its GUI.
It may also be possible to expand the existing partition but there is a risk this will break the installer and you'll need to start over.
"Create multi boot USB stick/hard disk with whole iso/s on any partition supported by grub2 keeping existing data untouched."