Cannot Copy Files To My Sansa Express

I am unable to copy files to my Sansa Express, as I get an error message that says, “The file (filename) could not be copied. It may not exist or is being used by another program”. When I try to sync my Sansa Express with Windows Media Player, I get an error message from Media Player which says, “Windows Media Player has encountered an error”.  What is the problem? I have already copied 540MB of music into my Sansa Express. I am unable to add more. I am scared to to a Firmware upgrade. Can someone help me out?

Firmware upgrades are usually safe.  You can also try formatting the thing through the Settings menu.