From the documentation at http://developer.android.com/reference/android/os/Environment.html (see the Constants section at the top):
String MEDIA_BAD_REMOVAL getExternalStorageState() returns MEDIA_BAD_REMOVAL if the media was removed before it was unmounted.
String MEDIA_CHECKING getExternalStorageState() returns MEDIA_CHECKING if the media is present and being disk-checked
String MEDIA_MOUNTED getExternalStorageState() returns MEDIA_MOUNTED if the media is present and mounted at its mount point with read/write access.
String MEDIA_MOUNTED_READ_ONLY getExternalStorageState() returns MEDIA_MOUNTED_READ_ONLY if the media is present and mounted at its mount point with read only access.
String MEDIA_NOFS getExternalStorageState() returns MEDIA_NOFS if the media is present but is blank or is using an unsupported filesystem
String MEDIA_REMOVED getExternalStorageState() returns MEDIA_REMOVED if the media is not present.
String MEDIA_SHARED getExternalStorageState() returns MEDIA_SHARED if the media is present not mounted, and shared via USB mass storage.
String MEDIA_UNMOUNTABLE getExternalStorageState() returns MEDIA_UNMOUNTABLE if the media is present but cannot be mounted.
String MEDIA_UNMOUNTED getExternalStorageState() returns MEDIA_UNMOUNTED if the media is present but not mounted.
It returns one of those constants.
getExternalStorageDirectory()
returns the path name to the device.