What is the speed of hard disk actuator arm?
You have two helpful measures for this:

track to track seek time and full stroke seek time.

track to track seek time will indicate how fast the head can move to an adjacent track on the disk. To get a velocity figure out of that, you need to know the track width.

full stroke seek time makes things a little easier, as it is the seek time from the innermost to the outermost track of the disk. With this value we can find an approximation without knowing all the details.

So this website claims a full stroke seek time of around 15 to 20 ms.

 

I approximated the length of the head assembly to be roughly 3 cm. And the length of the traveled arc approximated by a straight line is around 2.2 cm. So the traveled angle is 43°.

So the angular velocity should be in the order of 2150 to 2900°/s or 38 to  50 rad/s. 

It could probably be faster if you don't need to position it so carefully like it is done in a HDD.