Even the ones with a fan inside have hot spots because they are usually badly designed because they're trying to wedge everything into the smallest case as possible.
Yet another reason to get one with a built-in PSU: they seem to only be available in the 5.25" drive bay size, which leaves lots of free space around a 3.5" desktop harddrive.
The HD-only ones with external PSUs can be much more cramped.
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