If you use Format from a boot disk on the hard drive, it will make all data on the drive inaccessable to all but the most determined person. You can also go to the following link and download a program called Wipe from the section called Cleaners section. Format only ends the association of the written date to a particular pattern(file/directories) while Wipe will write "0"s to the entire drive making it less likely to have previous data recoverable to most everyone.
I would never donate my hard drive with my computer. Hard drives are so cheap, it's not worth risking my privacy. Physically destroy it by shattering it and melting it, and then buy a 5GB or 10GB hard drive for $30.
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