Everyone is replying as if the OP is asking about normal live environments, but I think he’s asking about having Linux actually fully installed on a usb instead of the ssd. In that case, most of the replies don’t apply. However, Mint has a way to install itself properly on a usb drive. Boot with the burned iso, insert a second usb drive, unmount it, and then install on it (you choose it during installation). It has to be unmounted first.
Can’t wait, it’s becoming really usable (I always needed adjustment layers, and it now mostly has them). I wish they offered an appimage though, I’m not big on flatpaks due to size.