USB stick no more recognized
On a new USB stick I have successfully installed a bootable Ubuntu from a Win10 PC with the tool LinuxLiveCreator. I succeeded in booting the computer from the stick.
Then I installed a Xubuntu on the same stick, after formatting it (with the same tool). I got no error message and everything seemed ok, but none of 2 computers with which I tried found a bootable system on it.
If I insert it in the Win10 PC it appears in the Explorer, but when inserting it I am invited to put a storage medium into the USB slot (already occupied by the stick).
On a Mac and on a Linux computer it doesn't appear at all.
What could be the problem? How to solve it? Ar there special tools to analyze and repair such a stick?