Sony Walkman NWZ-S638F Cannot add any more files to the device. It provides the message "There is not enough free space on the device". However there is are still 5GB of free space.

Kevin Thompson, September 2010

If the file you're adding is 4GB or larger, it might be that the software is using FAT32 formatting. As it's a Sony Walkman, you probably can't reformat it to NTFS (a Windows formatting), so the best idea would be to break up the file with a compressor. If this isn't the problem, then I have no idea.
Hope this helps.

{+S+}, March 2013