Does the Mac card reader show subfolders within album folders named MACOSX? You can delete any of those folders from the card–that’s where the ghost files are. They start with dot underscore and have names like ._01.Track 01.mp3. But I don’t know if Macs will actually show them to you unless change an option somewhere to show hidden files and extensions.
Also, before you reformat…
There can also be a problem with the tags of mp3 files–the electronic labels in the files. Some people get all clever and put giant album art in .png files in tags, or put huge amounts of texts or photos into Comments, and those can stop the poor little Sansa in its tracks.
There is a Windows program, mp3tag, that’s really good at fixing those. I don’t know of a similar tag editor in the Mac universe but maybe there is one.
If you have a friend with a Windows computer who’d let you install a piece of free software, search mp3tag and “View Extended Tags” in this forum to see the fix. And at mp3tag.de, make sure you download the mp3tagsetup(version number).exe file, not one of the green Download buttons.
But make sure that before you connect to the Windows machine, your Sansa is in MSC mode under Settings/System Settings/USB Mode. You can leave it there, even it was in Auto Detect, because that’s how it connects to Mac anyway.