Of course, the same can be said about Internal HDD where you can connect it to a different computer\'s HDD. The biggest downside is of course, inconvenient.
Imagine you want to transfer files from your computer to your brother\'s or your friend\'s computer or vice versa. If you use an external HDD, its more convenient since you can just copy the file from your Internal HDD to your external HDD. Then bring your external HDD to your friend\'s house or your brother\'s room and send all the files over his computer.
While with Internal HDD, you have to open up the computer\'s case while it is off, and unplug the IDE and powe cable, then carry the HDD to other location, say your friend\'s house. Then, you have to open up your friend\'s computer case while his computer is turned off, and connect your HDD alongside with your friend\'s HDD via IDE cable(s) in the motherboard.
That could be somewhat inconvient and also the internal HDD is more fragile than external one imo. Which means you have to be more careful when carrying them around.