I am using Buffalo external hard disk ( model HD-CE500U2) . The firmware version is unknown. I use it on a lenovo T400 or a Dell Lat 630. Both have windows 7. One day i could not see the folders anymore. It still recognizes the drive. I have used about 250 gigs on it ....and it still recognizes that space as being used. I can even add new folders to the drive. Just cant see the old folders anymore. I can see the drive in disk management as well (FAT32 Health Primary partition). When i click on the drive in explorer ...all i see is "n" which is a system file and thats it ...please help !!!!
Unfortunately it sounds like a classic case of data corruption. I would suggest exploring data recovery (if necessary) and the formatting the unit afterwards.