Re: portable hard disks
All an external USB harddrive is.... an ata(ide) internal harddrive that is mounted into a hard case(enclosure) that is powered by a USB connection or a firewire connection.
Most external cases are designed so as to remove the hdd and replace it with another one if damaged.
The issues come about from incompatabillity between the USB supply and the drive itself. That is why it is always best to buy a prebuilt unit and preferably with it's own external power supply.
But is perfectly acceptable to put together a case and hdd of your own choosing.Just be sure to research for issues with the case that you are trying to use.