Like desktop HDD may not be designed to withstand lot of movements / shakes. But laptop HDD are better in that matter and WD Passport etc are made exclusively for that.
I am not sure about how they are built, but I can only share some views and personal EXP
and when things have to go wrong they just go wrong
i mean I have dropped the external HDD from stairs couple of times
and in couple of time they have been some hard fall
but the HDD and the case withstand the fall !!
how ever, once I dropped my Portable lappy SATA drive 160 GB, which could be seen in the picture, from just 2 feet high bed to the room floor in a hotel where the floor had thik carpet in it,
but the poor thing stoped working
So all depends
Personally with my exp I would say Notebook HDD + Casing is really portable
but costly when u need bigger space
Desktop 3.5" HDD with casing is cheapwhen u compare with the space, as one may get 320 GB Portable @ Rs. 3600/- + TAX where as in that price range u can only manage a 160GB Lappy HDD + Casing, but 3.5" + casing are really bulky and will need extra power....