From your description, it sounds like you have a corrupt drive. This can happen a number of ways either through physical shock - i.e. you dropped the drive, lost power whilst it was writing data or you might not have ejected the drive properly before disconnecting from the pc.
You may have some luck exporting some of the data from that drive onto another through multiman (seeing it's reading it), but be careful as you may be transferring corrupt data.
If as I suspect you drive is corrupt, there is unfortunately little option other than to reformat. I know the feeling, I had a 640GB full of games, homebrew and saves die on me recently.
The only slight glimmer of hope that you might have that I did not have with an internal PS3 drive is that you might be able to source some disc recovery software (if you're canny enough), that might be able to recover some files.
I use a mac and I have an excellent disc recovery program but I wasn't able to use it because the internal PS3 drive is encrypted..
"We all agree your theory is crazy, but is it crazy enough?" - Neils Bohr