Agreed.On the point of External HDD (Internal HDD preassembled with an enclosure), I personally stay away from those.
You don't know what type of HDD they've put into the External one.
Additionally, while you can sometimes find out what drive is inside, manufactures will change their warranty terms simply because of how the drives are pre-packaged. Case in point: Western Digital MyBook (3.5") and Passport (2.5") pre-packaged external drives, which use WD Caviar and Scorpio internal drives that are available separately as internal bare drives. The difference however is that the internal bare drives come with a full 3 year warranty, while the pre-packaged external drives have limited 1-3 year warranties -- even though the physical drives are the exact same.
When the WD Passport drives first came out, they were being sold noticably cheaper than the internal bare drives. People found out quickly that the actual drives were the exact same and started buying the external drives and gutting them (to upgrade their laptop hdds) -- only to be dismayed later when they were left without warranties.