Not Storing Backups on an Internal HDD

I've often read, in these boards and elsewhere, that one of the reasons for not storing backups on your computer's internal hard drive - even a second internal hard drive - is because 'it's not a question of if your hard drive will fail, but when.'

I don't quite understand the logic behind this, and here's why:

1) If I store my backups on my external hard drive...well, that's also a hard drive. And if I store my backups on a network...well, that too is a hard drive.

2) I've been reading for years that hard drives are getting more and more reliable, to the point where the chances of a drive failing on its own are very, very small. And from what I've read, hard drives are a much more reliable (and longer lasting) medium than are CDs or DVDs.

To my understanding, the reason for not storing backups on the same computer is to safeguard against the relatively more likely possibility that some external cause (viruses, theft, lightening strike, etc.) will make the computer unusable. Is this correct?

Daddy
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