Same thing happening with me. I upgraded to 2.4 and now any new fseq files don't show up in file manager. I made sure to set to the USB like above, but still no fseq is showing up in the file manager. I only have 12 songs in there ranging from 20MB to 60MB and I have a 16GB usb connected and set as external memory. Can't figure this out.
*EDIT I closed my computer for an hour, loaded it back up and now magically it works. lol