Settings/System Settings/USB Mode.
Try MSC.
Try MTP.
Your computer only sees one USB mode at a time. Imagine pipes to your Sansa. If it was sent through one pipe (USB Mode) the computer sees it. If it was sent through a different USB mode the computer DOESN’T see it. And you say you used a different computer.
Meanwhile the Sansa doesn’t care how it got there, it sees everything.
it was probably on Auto Detect and Auto Detect sends it to different USB modes.