Test the card with
http://www.softpedia.com/get/System/System-Miscellaneous/H2testw.shtml
Even if it is a good card, don’t try to dump everything onto it at once. Try 1G at a time, put it in the unit, see what happens. One bad tag can make the Sansa give up, and 20GB is a lot of tags. If you do smaller batches, you can troubleshoot if something goes wrong.
64GB may just be too much, though. What happens if you let Windows format it?
