Can't delete music from my Sandisk e250 2GB MP3 Player without getting error message on laptop!

Try browsing the file structure using Windows Explorer (rather than Windows Media Player).  Even in MTP mode you can find the player listed as a portable media player device.  Delete the files you no longer want on the player and use drag and drop to replace them.  Consider switching to MSC mode, formatting the player, and the reloading your media files.  You only need MTP mode for using DRM protected content.