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Generally, daedra hearts drop in the Cadwell's Gold zones, so the VR6-VR10 zones. You may occasionally see them outside of those zones, but they only seem to reliably drop in those zones. And the Daedric motif only drops in those zones, so it is kind of connected.
Deconstructing daedric gear can also yield a daedric heart every now and then, if you have maxed out that craft.
Knight-Captain of the Tamriel's Elite Sentinels ( EU server-PC)
"Courage is not the absence of fear, but rather the judgment that something is more important than fear. The brave may not live forever, but the cautious do not live at all."
Deconstructing daedric gear can also yield a daedric heart every now and then, if you have maxed out that craft.
Indeed, I managed to collect 4 daedra hearts before I even completed EP and arrived in DC from doing this. That was before I even knew I wanted them, was just putting them in my bank each time one randomly appeared.