To my experience several things might have happened to your external hdd.
1. Bad sectors
2. Bad SMART (IC spoilt)
3. Using the front chassis usb ports(for pc)- this is a big no no. Pls use the back ones as the front usb ports have not enough power to fuel the external hdd. data will corrupt.
4. Initial formatting of hdd should be formatted by a pc using a hdd convertor. Means the notebook hdd should be attached to a pc using a ide adaptor. The data will be more stable and correct hdd size will be reported accurately.