llancet:

The easy question first: Bad news--It's a costly operation. Generally, if you're forced to make the heads move, you're talking 5+ milliseconds (and the 5ms is probably a high-end drive).

The good news: Modern drives typically read an entire cylinder at a time, so if your seeks are mostly local, you can often bypass the head motion. (The more platters in your drive, the more information in the cylinder.) So when you preprocess the file, try to arrange things to keep the seek distances small, and you'll have fewer head movements.

When a head motion occurs, it has a trapezoidal motion profile: At the start of the movement, it accelerates to "top speed" which is a constant-time operation. At the end of the movement, it decelerates back to zero speed, which is another constant-time operation. After that, it takes a little time to stabilize the motion and sync up with the servo track. Between the start and end of the movement, the heads travel at a fixed velocity, so the more tracks you cross, the more time it will take. (However, I doubt it matters much, as I believe (no evidence) that the time is dominated by the speed up, slow down and synchronize operations.) (Note: I glossed over a good few details, such as short head movements in which case the speed up never gets to full speed before the heads start slowing down.)

Note: I'm not a hardware manufacturer, though I play one on TV.

...roboticus

When your only tool is a hammer, all problems look like your thumb.