Answer:
One of the reasons external hard drives appear to fail more than hard drives installed inside computers... is because there are a greater number of points where failure can occur.
In many cases of external hard drive failure, the point of failure isn't actually the hard drive. It is the USB to SATA converter connected to the hard drive inside the casing. To many users, this appears as failure of the drive itself... because they can't access it dependably or at all.
Then there is the power supply, in the case of external desktop drives. If that fails or becomes an issue, it could easily make the user think the drive has failed... and yes, I've had more than one customer ready to throw away a perfectly good external hard drive just because the little DC adapter for the drive was bad.
Explanation:
Hope it helps