The first is that the HDD has failed and if you really need the data off it you'll have to take it to a data recovery house and pay the price involved.
Second is that the USB Enclosure may have failed and needs replacing and that there is no damage to the HDD at all but that the USB Enclosure is dead.
The third option is that both the USB Caddy and Drive have died and this commonly happens through a lot of use as the drives do not run cool and the longer that they run in an external Caddy the hotter that they get and eventually fail because the heat causes the solder to melt and stop making contact, that's the best case scenario worst case is that the heat gets so intense that the Magnetic material on the platers lifts off and everything is impossible to recover.
These are not ideal Backup solutions and shouldn't be used as such as HDD do die on a fairly regular basis then you are up for quite a lot of money to recover your necessary Data I recently paid 46 K for this to recover some Tax Data for a customer who was undergoing an audit. Needless to say they no longer rely on an external HDD for Data Backup but continue to use these for an easy fast method to transfer data between different computers in different locations.
If you need a Data Recovery House look in your own area as these tend to be faster in recovering your data but I've had excellent experience with On Track and while I've never had any direct experience with them Seagate now has a Data Recovery Arm that you could try
On Track is available on the Web here
http://tinyurl.com/phlya
Seagate is available here
http://tinyurl.com/ysuh6g
You can also try a Goggle search to find a company in your area so that transport times are diminished if necessary.
Col
