Where do you get that from? A hard drive is a hard drive, there is no physical difference between an internal and an external when it comes to the head and platters.
It's common knowledge. Many external enclosures are simply not designed for the HDD to be used continuously, mainly do to poor cooling of the drive. Additionally, while the HDDs in external enclosures are indeed the same as their desktop counterparts, operating systems do treat them differently. For instance, the onboard cache of the HDD is generally not used, because if you disconnect it, you risk data corruption and loss. The lack of the usage of the onboard cache means that when you write even small files to the HDD, it means that it will literally be written to the HDD, instead of the the cache (which is typically 16/32 mb of memory depending on the HDD). This causes the HDD to work "overtime." Over a short period of time, this can cause the external HDD to heat up, especially when working with files that you are saving changes often. Having the HDD overheat will severely limit its life.
For example, Vista has a resource monitor that you can open up and examine disk usage. Looking at it, I see several processes that are writing to the HDD, however, the HDD activity light never turns on. This is because the HDD's cache is being used. If this was running on an external HDD with the cache disabled, the light would literally constantly be on, because the HDD is constantly being written to.
However, Windows itself does have an option to enable the cache on plug-in play external HDDs, but is disabled by default. I assume that Mac OS X is the same way.