>In the middle of a transfer into a USB key, I got a message files were no longer being able to be copied. When I try to do another test, I now have a message stating the disk is protected. What can cause a USB key to become protected by itself?

I can't tell you exactly what causes it, but in the cases where I've had it happen I'd suspect a problem with the device (e.g. storage block that went bad). In those cases I was able to write to it again by low-level format using a (manufacturer-provided) tool -- though in most of those cases I had the same problem (or worse -- write-protected *and* having zero bytes of storage) within a couple months (especially if I try to fill it near capacity). It is apt to occur if you're using the thumbdrive in a "sneakernet" situation where you're frequently writing data on it (obviously this includes situations where you're erasing data -- which *does* involve writing data).