now this is common with external hard drives & even sometimes with partitions.
However, if you format the unrecognized drive, you can still recover old files on the disk, as zero's are not written through out the disk/sectors, using softwares like, Final Data, Pc Inspector File recovery & so on.
If it is a hardware problem you can try the old myth of putting it up in a freezer & trying to run it before it warms up & defrosts. Worked once for me though.
There are only a few specialists in dubai who can play with the internal disks without damaging it & recover your stuff, others are all bluff, lets say the folks in khaleej center.
try them:
www.drd.ae , i heard they are good.
best of luck