I would like to use my USB stick for my data, while keeping a bootable live copy of Ubuntu on it for the emergencies. 

I think that until some versions ago the StartupDiskCreator would create a bootable drive that I could still use to store other (non OS) data. 

Currently, I am left with two write-only partitions, big just enough to contain the OS, and plenty of spare, unformatted, space: see screenshot from the Disk utility

[![Disk utility](https://i.stack.imgur.com/xclel.png)](https://i.stack.imgur.com/xclel.png)

Can I create a partition in that free space and use it on other Linux, Mac, and Windows computers (I'll make it a FAT filesystem)?

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(There are other answers regarding this, but they are from before the Live USB sticks started showing the present behavior)