to take into account the new default of starting the first partition
after the boot blocks instead of at sector 0. If you used automatic
sizing when the first partition did not start at 0, you would get
an error that the automatically sized partition extended beyond the
end of the disk.
Note that there are probably still many more complex cases where
automatic sizing and placement will not work (e.g. non-contiguous
or out of order partitions).
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