Why are these hard drives getting corrupted?

Yep, I agree with mayer. To prevent these issues in the future, make sure that you are properly ejecting your drives, shutting down your laptop/computer properly, and keeping the external in a place where it won't get banged around.

If the externals are still having problems after formatting/proper usage, you might want to contact Western Digital for their warranty, which I believe is 2-3 years per HDD.

Make sure they are getting enough power,thats normaly the no 1 problem.make sure you use a powered hub,try not to use the double usb lead that comes with them,they realy dont source more power to be honest,they are a wast of time,make sure the usb lead is of good quality and nice and thick too.