You may be legally required to destroy your hard drive
There are laws that require you to prevent unauthorized access to personal information.
Over the last 2 years alone, there has been in excess of $10,000,000 in fines for improperly discarding sensitive personal information, not to mention scores of embarrassing headlines.
The legal requirement for data breach notification can cost tens of thousands of dollars and a whole lot more in lost customer confidence and future business.
You have a ethical obligation to your clients and stakeholders to properly destroy personal and proprietary information when discarding or transferring ownership of old computers.