I found this interesting information on the German support, and I think it may interest English users.
When I installed Slitaz on a USB key the result was strange :
On boot, the first option of the menu did not work : Countdown was looping indefinitely, and when I hit Enter key countdown restarts. The second option (core) was working, but login failed and it was necessary to use root / root.
The German user pointed that this happens if the key is formatted in Fat32. When formatted in ext3, everything works fine. I tried and it worked.