It's theoretically possible to move the user changes and home partitions to a USB key, however there would be a lot of risk of lost data/failures, as USB keys are somewhat unreliable. I'm not sure this would all work, but if you want to try it, bear in mind that the user changes and home partitions are mounted by e2fs label USER and HOME respectively. So what you would have to do is use something like gparted to create 2 linux partitions on the key and format them ext4, with the labels USER and HOME. Then you'd need to boot ubermix in generic+single user mode, turn swap off (swapoff -a), umount /home, fdisk /dev/sda, delete partitions 2 and 3 on the disk, and write your changes. Next, insert the USB key, reboot, and cross your fingers. No promises.