If I'd like to install Trisquel (or any other distribution) on a USB storage
device, I'd first prepare two USB storage devices. Create an installation
media using one of them and then use said installation media to perform the
installation to the other (destination).
The process of installation is essentially the same as normal installation
(to internal hard disk/SSD). Just keep in mind that you need to install boot
loader to the correct device.
Non-journaling file system such as ext2 is recommended when you install the
operating system to a USB storage device.
Finally, I'd like to save most of my important data on a (magnetic) HDD
rather than SSD. When the SSD fails, it's much harder to recover the data.
