From this link I think it says files are written to single sectors on each platter.
http://computer.howstuffworks.com/hard-disk6.htm
I have heard defragmentation helps move files to the outside of the platter for faster retrieval. So in theory the machine should be filling the outer portions of each platter and returning to the first platter to begin filling the second outermost ring. Which platter is used first?
Very interesting thread and I hope to read the real answer here.    

"defrag" is just putting the fragmented files back together

when using windows the system writes files in what ever space it can find which in turn causes the fragmentation, hence why defrag needs to run every so often to put the files back together...

as for the writing issue its usually across the platters at the same time, system usually goes from inner most to outer most...but it goes so quickly that the data can and usually will end up anywhere and every where as its being written, accessed and all that fun stuff...