I'm wondering why how close a permanent magnet can get to a hard drive before the field can cause damage to the data.

Every hard drive contains a strong magnet that operates the arm for the pick up heads and the field next to the disk is quite strong. However, the field must not have any effect or there would be immediate evidence of damage.

 

I believe the field from that arm magnet is fairly closely contained as there are two magnets which generate a very localized, closed circuit magnetic field the the arm magnetic coil works with.
An external magnet does not have a closed loop and thus could likely affect the magnetic disk from a significant distance.

 

Has anybody ever heard of a hard drive being accidentally messed up by having a magnet placed nearby? If yes, what are the details?

 

Has anybody ever heard of a hard drive being accidentally messed up by having a magnet placed nearby? If yes, what are the details?

Not likely with even strong magnets.
https://www.kjmagnetics.com/blog.asp?p=hard-drive-destruction

How strong a magnet? I got a several Tesla beam analyzer electromagnet I could test a drive with. It might come back in little pieces.

 

I asked the question because I had a foreign object stuck in my keyboard and I used a flashlight with a magnetic clip to light the area.

However, I kept the magnetic side at least 8 inches from the computer case. Obviously, if it caused any damage, I wouldn't be using my computer to post this!!!