Try this, it worked for me, it seems to be a common problem with the SD cards, but it's a little involved, and must be done every time before you mount the SD card.
Please Note! Make sure before you start your Droid is not connected to your computer.
You need to uninstall the drivers each and every time you want to mount your SD card, but in order to uninstall them you need to be able to uninstall hidden devices in your device manager.
In order to uninstall hidden drivers do the following:
(Note: this step must only be done the first time, it allows you to view hidden drivers permanently)
1) Open the