hard drive head/seek motor with Arduino?
As snoopen says - you want a "voice coil" driver or actuator.
It's well within the capabilities of an Arduino to drive one of these.
There are a number of people offering on-web examples of HDD voice coil drivers used for other purposes.
Wikipedia overview
This provides a general view of an HDD voice coil actuator
This is an actuator withy the magnets in position above and below the coil
and here is a unit with the magnets moved out of position to provide an internal view.
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Here is an utterly amazing LASER shutter driver achieving mechanical shutter rise times of
500 nS !!!. There are a number of links off this page to other variants and upgrades.
Their basic driver circuit is suitably simple -
Here is a formal 2004 paper based on the above
And three years later in 2007, an updated version
Here is a throttle position control using an HDD actuator.
Circuit diagram here - the actual actuator control is the middle third hozizontally and the actual driver is 2 transistors driven by an opamp. An Arduino could easily feed a s signal to such a circuit. eg smoother PWM or a DC voltage generated in some other manner.
I was going to post another comment but I was writing too much. So here's the beef:
You're looking to control what's called the voice coil. There's a little information here which should get you started: http://spikenzielabs.com/SpikenzieLabs/Laser_Display.html
In response to your comment:
Yep, the examples I've seen use an h-bridge circuit in locked antiphase configuration. The digital signal in this case switches the h-bridge output between -5v and +5v. You will need to tie the enable pin of your h-bridge to always be on during operation. You should then be able to generate a square wave tone on the Arduino and use that as the control signal. Be wary that the voice coil might not like to be held in one position for too long so you might need to drive the enable pin off when you're not sending a signal.