External drives are usually just encased internals, so the reliability and failure rate will generally be the same, assuming of course that the external drive isn't going to be moved around much. In practice, external drives are more likely to be exposed to knocks and rough handling, which can significantly increase their failure rate.
I think incorrect handling has a large part to play in the early demise of most mechanical drives. Modern hard drives rely on precision engineering and extremely close tolerances for the heads to correctly float and the platters to spin at high speed with minimal vibration and resistance. Any significant jolt in the early life of a hard drive (such as it may receive from a
careless courier or air-freight handler) that isn't sufficient to cause the drive to be DOA, could still result internal movement and/or deformations that cause greater strain and wear, resulting in a shortened service life.
I think this is probably why there's such a difference of opinion and experience when you ask about hard drive failure rates. Over the many years I've built and repaired PCs, I've bought hundred - and worked with thousands - of hard drives. In my own experience, apart from a few periods where batches of hard drives (like the notorious IBM/Hitachi Deskstar drives, for example) had known design/manufacturing flaws, I find that premature failure is quite rare. I also find WD to be one of the most reliable makes of HDD, contrary to what some people seem to be experiencing. In fact, I buy more WD HDDs than any other make. If I had to pick a make of HDD that had the most failures, for me that would be Seagate, though the difference in reliability is marginal and more to do with a recent spate of failures rather than an all-time failure rate.
Also, in the majority of cases, I find that data is recoverable -- though not always, so you should always keep backups of course. In my experience, most drives fail partially or slowly and, if the symptoms are spotted early enough, 90%+ of the data is recoverable. In the cases where the failure is sudden (typically resulting in a drive that the system no longer recognises), the cause is often firmware related. In such cases I've usually managed to recover data (when necessary) by resorting to flashing the firmware directly at board-level.