Assuming you used the Panasonic formatter, and that you always use the "safely remove hardware" utility before you remove your microSD card from your PC, the problem could be a bad microSD card, a bad microSD card reader, or a bad DSTT.
Try it with a different microSD card and/or card reader if you have one. Or if you have access to another DSTT, try your microSD card in another DSTT.
Given the large number of bad DSTT's that have been reported lately, there's a good chance that it's your DSTT. If that's the case, you should contact the store you bought it from and replace it.
-Bri