In case anyone else finds this thread...
We just had this issue as well.
If you open disk management (start, run, diskmgmt.msc), look for a drive that is "removable", that has a drive letter attached to it., but doesn't actually have any media.
Right click the Disk # on the left hand side of the screen, and remove drive letter.
If you actually do use a SD card reader, or other similar devices that need that drive letter, go find a program called "USBDLM". There is a setting in that utility that will attach the drive letter when media is present, and remove it when the media is removed.