If you transfer lots of music and files from your PC or Mac to your Droid 2, you may eventually run out of storage space. It’s a good idea to regularly check how much space you’ve got left on it. To do so, from the Home screen press the Menu key and tap Settings→“SD card & phone storage”. You see the total amount of capacity on your SD card and how much storage you have free.
If you’re running out of space on your SD card, delete files from it. If you’re deleting music, use the Music app rather than the Files app, because if you delete the files using the Files app, the Music app will still show the music as being present, even after the files are gone.