install lubuntu to a USB

I suggest that you install mkusb according to the following links,
help.ubuntu.com/community/mkusb
help.ubuntu.com/community/mkusb/persistent
Then, with mkusb installed, you can create a persistent live drive with all current versions of Ubuntu (also 64-bit versions for newer computers).
See also the following links for more details,
how-do-i-make-a-persistent-live-usb-of-ubuntu-16-04
unable-to-boot-ubuntu-live-usb-flash-drive-with-casper-rw-persistent-partition

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Edit: After a debugging dialogue, a bug was found and squashed. The problem was some confusing data due to a floppy drive (a bug that only affects very old computers).
A new version can be installed via the following commands

sudo add-apt-repository ppa:mkusb/ppa
sudo apt-get update
sudo apt-get dist-upgrade

The current update (2017-04-03) installs mkusb version 12.0.9