Couldn’t find this on any forum on t’internet, so thought I’d post it here just in case it helps someone.
If your tomtom go 720 won’t find MP3 files after having copied them onto either the onboard flash disk, or an SD card (IMHO the most sensible thing to do), then there’s every chance you’ve been stuffed by the new ID3 tag formatting. The new format just simply isn’t recognised.
The solution? Download mp3tag and use it to remove the new ID3 tag, leaving behind a version of ID3 tag no more than version 2.3; this is the version of ID3 tag that I’ve currently got, and all MP3s are recognised by the tomtom. It’s a bit of a pain in the neck, but at least there is a solution.