An external drive sitting next to your laptop is just as easily stolen as is the laptop. That drive only exists to save your data in case of a fail of the main PC drive.
Once in my possession, there is nothing you can do to save the data. Maybe hard drive encryption, but then I'd just toss it out.
Erase, format, delete partitions...if I get pissed off enough, toss it in the trash. Same outcome - *your* personal data is gone.
Solution? Save another copy offsite. Dropbox, at the office, at dad's house...wherever.