The AFG3000 recognizes the USB memory stick but does not see the files. Why?

The AFG3000 recognizes the USB memory stick but does not see the files. Why?

A possibility is that the memory stick is incorrectly formatted. The format needs to be Fat32, Windows format. Other formats such as NTFS will not work.

Also the AFG3000 will not show files which do not have the correct extension. i.e.   .tfw (trace data files) , .tfs (AFG3000 setup files)  .Tfs files are only viewable under the Save / Recall menu,  while .tfw files are viewable only in the Edit menu.