According to Official Guinness Records,
The highest voltage survived by an external hard drive is 75,000 volts, achieved by the Threx 2 Datasafe manufactured by secumem Gmbh and tested at the University of Applied Science, Munich, Germany, on 4 December 2009.
The 75,000 volts survived is across the connectors (where the greatest likelihood of electrical overload exists), however it can deal with even greater load across the casing itself - in simulated 'lightning strike' tests, the device survived 240,000 volts without loss of data. The Threx 2 can also withstand 1 hour of a high intensity fire.
For a complete list of 2009 records, please visit 2009 Guinness Records in Germany.