You could have problems with an external hard disk, such as safely removing it (even after unmounting it). In this case you could try this code:
sudo hdparm -Y /dev/[your device: something like sde1 or sdh1]Another problem could be the impossibility to mount your external hd. In this case, if the hd has an ntfs filesystem, you could try this code:
sudo ntfsfix /dev/[your device]I found this last code on www.makeuseof.com.