Yes, the format was the problem. You need FAT32. I don’t think the Clip reads NTFS and it definitely doesn’t read eXFAT.
For others who may have this problem, depending on your version of Windows you may need third-party software to format to FAT32.
And if you fill up that 64GB with mp3 files you will probably exceed the database limitation on the Clip, so you might want to turn to Rockbox.
