I recently bought two Hitachi HLSMPUA3201ABB external Hard drives. They come with a USB key attached to the enclosure with a magnet. It's not a very strong magnet but I can't help wondering whether the drive would last longer without it. Should I consider returning it?
It won't damage your HDD, don't even give it a second thought.
See here for reasons why.
The only magnets powerful enough to scrub data from a drive platter are laboratory degaussers or those used by government agencies to wipe bits off media. "In the real world, people are not losing data from magnets," says Bill Rudock, a tech-support engineer with hard-drive maker Seagate. "In every disk," notes Rudock, "there's one heck of a magnet that swings the head."
There are magnets inside the hard drive also and are not shielded in any way.