If it's unmounted, no writing operations are taking place. There should be no danger in unplugging the device.
Remove the USB connection first. This usually causes the drive to spin down, in my experience.
PC: LMDE x64 MATE/Cinnamon, Kernel 3.9, Nvidia GT620, i7 3770K, 16GB RAM
Laptop: LMDE x64 MATE, Kernel 3.8.6, i3, 6GB RAM