If you are still looking for a solution, here are some suggestions:

If your player has been set to MTP mode (Settings>USB Mode>), your new micro SD card will not be visible. To gain access to the card, change to MSC mode. Don't forget to switch back if you download protected content from Rhapsody, Netlibrary and others.

Another possibility is if your card is larger than 2GB. Although I don't think it appears in the Sansa tech manual, I recall reading on cnet or some other site that the maximum card size accessible by the e200 series is 2GB.

On very rare occasions, I have had to format a card to be able to read it. I would not recommend this until you can confirm with another method of accessing the card that it was not readable.

Perhaps you have already solved the problem...if not, hope this helps.