this is tru in gnome! (I've noticed the same thing happening).
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Nautilus needs two tries to open a folder the first time linux starts
https://bugs.launchpad.net/bugs/611670
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I guess it mounts the partition lazily so that won't take time at
startup. And thus first the mount of that certain partition is
created...but forgets to open the viewer...:) (my noobish explanation
found of this behavior)
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Nautilus needs two tries to open a folder the first time linux starts