I believe it is the live system. Isn't your USB key 8 GB large?
Launch GParted (which is on the Live system). A password will probably be asked: try to directly type Enter and, if it does not work, try "trisquel" (I never remember which one it is). In GParted, try to understand how the Trisquel install has modified your disk partitioning. If you kept the default partitioning scheme, there should be a small linux-swap, a rather small (14 GB if I remember well) ext4 partition and a rather large xfs partition. Do you see them?
