Hard drive's start on the outer track of the platters and write their way in, yes. Although it's not just a straight run across all the platters simultaneously. I think Maxtor or Quantum have a neat little explanation of how data is written across the platters.
The outside edge of the platter is faster than the inside, and since the density is the same across the disk, this means the outside edge passes more data across the heads in less time, which means the outside edge can stream data faster. So if you have multiple partitions, the one on the outside will be the fastest, and they get slower as you move inward.
CDROM's read from the inside to the outside. The older kind of drive used CLV, constant linear velocity, which meant the same amount of data passed the read laser all the time, so you had a drive that was the same speed across the disc (it would slow down the spinrate when it got to the outer half of the disc in order to keep the data rate the same). Newer drives use constant angular velocity (CAV) or a combination, so that it always turns at the same rate. So when the read laser reaches the outer part of the disc, the data is running past it at a faster rate than the inside of the disc (same density, but the outside of a circular spin goes faster than the inside), so you get higher speeds on the outside of a CD than the inside. That's why you have '32X MAX' drives. They spin at a constant spinrate, say 200RPM's (random number). On the inside edge, that makes it maybe a 12X data rate. As it moves outward, the datarate increases slowly, so halfway across the disc you're at maybe 24X, and at the very edge you're at 32X. However most CD's aren't full, so it's rare that you ever reach the 32X portion. More often you get far less than the max rate.
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