I don't believe it, not at random anyway. Especially the missing "backup data on the usb drive" also missing. I can see it happening from ONE image, but not all 4, along with backup data.
Make sure you didn't leave a connection LIVE to DreamboxEdit or the like FTP software. Though that would MAYBE only be capable of causing data from a single image to vanish. Whichever one you are booted into.
IF you are playing around with Network Neighborhood connections to the Mio, be exceptionally careful doing that. It's easy to nuke or accidentally move things around you didn't mean to. I did that myself, but it wasn't random. Except once again, it can only happen to whatever image is LIVE at that moment, not all of them.