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Rotational delay is the time necessary for the drum to rotate until the right sector can be reached by the head assembly. Seek time is the time necessary for the head assembly to move in the right place.
If they moved at the same time, only the largest of the two should count as access time (usually the seek time, I believe). But every text I read about the subject states they have to be added – which would imply that they cannot move at the same time.
So which is correct? Sum, or maximum?
The head may get into place, but the disk might not be in the correct position at that point. So imagine that first the head has to move, then wait for the spindle to rotate. It might be there right when the head arrives, but odds are it will have to wait at least half a rotation for the right sector to be reached. So summing them allows for both.
Edit:
So imagine it like a Merry Go Round. You may be able to run up to the merry go round in 2 seconds (you being the head). But you might have to wait a good 5-10 seconds for your specific horse to make it around to you after you arrive (horse being the sector you want to access).