That is weird and is usually due to the drive not being formatted in a windows format. As you were using it yesterday fine, it could be that Vista has gone crazy, have to tried restoring to a point before yesterday? If so and the issue is still there, grab a partition manager and check to see if it can see the space. If it can, then try and repair the partition table and prey it works, because otherwise you'll have to reformat it
crabby