im not a big mac person, and i am not familiar wtih that specific hdd enclosure, but a few things that can cause your disk to go bad are power surges from the power going out while its switched on, and not properly disconnecting it and powering it off can also damage the disk
chances are the index/file tree is corrupted and your data is safe, but the computer just cant access the index to find where it is at. this can also happen if you have a huge number of indivivdual directories (i.e. if you have mass music albums). try to find some good recovery software if you dont haev any luck, i use getdataback ntfs but i have a pc so im not sure what to use for mac
hope that helped