Could it be that the Clip+ doesn’t support 64gb cards?
Assuming that it does, perhaps you could try deleting the file/files that the Clip+ uses as a database. I don’t have a Clip+ myself, but if it’s anything like the Fuze, perhaps the name of this file is “MTABLE.SYS”. It couldn’t hurt, since the file will automatically be rebuilt from scratch.
Could it be that the Clip+ doesn’t support 64gb cards?
It doesn’t in its native exFAT (SDXC) format, but it will if re-formated to FAT32 (SDHC). Many have successfully done it and are enjoying the extra memory space. But it seems to depend on what formatting software you use; some will convert it properly while others won’t. Gui Format is one I’ve heard works well for this.