Is because the HDD gets unplugged without using the Safely Remove Option. On any HDD there should be a Partition and some slack unused space if there is 1 partition on the drive of course if there are more made there will be more depending on what they are and how the drive was formatted.
Anyway if you can see Files in there but not access them I would tend to think that the Partition Tables/Master Boot Record whatever you wish to call it has been trashed and you need to use a Data Recovery Application to rebuild the Partition Tables. On Tracks Easy Recovery is the best for this job but it's also very expensive so it's not the use once and throw away type thing, though it is considerably cheaper than the cost of a Data Recovery House, Depending on how willing the person is to pay for recovering his Data you may be able to =buy the On Track Application but you can use the other type of Recovery Software which recovers the Files from the HDD after a Partition has been made.
If you want to search for a suitable Data Recovery Application here is a short list of the more common ones available.
http://tinyurl.com/273ay5
http://tinyurl.com/rmvrv
http://tinyurl.com/yvrsl
http://tinyurl.com/yyk8cm
http://tinyurl.com/382vm
http://tinyurl.com/ynwnel
http://tinyurl.com/nc4kx
http://tinyurl.com/2wox7l
http://tinyurl.com/374d8b
http://tinyurl.com/djw6u
http://tinyurl.com/f73se
http://tinyurl.com/8bd3f
http://tinyurl.com/afu2j
While the last 2 are from the same company they are very different products. Win Hex should only be considered foe use by someone who knows Hex Inside Out while Davory is a great cheap General Purpose Application.
Personally I prefer the On Track Option but it's anything but cheap. However because I use this type of software a lot it's paid for itself many times over by now, for a cheaper option Spin Rite from Gibson Research or Stella have both very good reputations by other TR members.
Col