Hard Drives blow my mind...

I recently got an external harddrive that I have hooked up to my Airport Router, so thereby either of my computers can put stuff on it. I have an iTunes library on the drive which I have used to free up space by moving all the TV shows off of my laptop.

Tonight I happened to be listening to iTunes via this library, and copying over a new video from my laptop to the harddrive, and I thought to myself... how does it do both of these things at once? I think of hard drives as really really really sophisticated vinyl record readers and writers. So if it has one head, how does it do both of these things?? Clearly they can, as you never think about it when its the harddrive IN your computer. HOWEVER right when I was thinking of this, the "write" job to the drive failed. I fooled around for a few minutes, and found that the only way it would write the video files over without errors was if there was nothing else being done with the drive....found this sort of odd. I didn't expect my rudimentary picture of a vinyl record hard drive to really explain much in the real world....

So what's the deal? How do harddrives read/write from one part of the disc, while also read/write from another part of the disc? Maybe mine can't do this very well, but it's much more likely to do with the fact that its going over wireless, then through my router, and then finally into the drive.....