USB Flash Drive, Memory Sticks, FOBs, whatever you call them. We all have them. Some of us have lots of them.
And they are absolutely NOT safe for your only copy of anything that is important to you.
They fail. Actually fairly often. So don't keep any documents or other files on them unless they are copies of files elsewhere. If files are important to you, they should exist in at least 2 places. Sure you can use them as primary storage for some files. But again, have copies elsewhere.
This public service announcement has been brought to you by me.