Nothing can more wrong than assuming this.
HDDs fail !
This is since, and there is a good theory behind it. Each HDD producer has a MTBF – Mean time between failures.
Mean time between failures (MTBF) is the predicted elapsed time between inherent failures of a system during operation. MTBF can be calculated as the arithmetic mean (average) time between failures of a system. The MTBF is typically part of a model that assumes the failed system is immediately repaired (mean time to repair, or MTTR), as a part of a renewal process. This is in contrast to the mean time to failure (MTTF), which measures average time to failures with the modeling assumption that the failed system is not repaired (infinite repair time).
The problem with HDDs is that they can’t usually be repaired. So, this is the Mean Time to Catastrophic Failure or Mean time to fail.
Do backups on more than 1 HDD because this is the only way to reliably make sure that your data is stored safely. If you can afford, cloud backup like Dropbox is a good solution as well.
Constant failure is the reason why we have RAID 0-6. The more disks you have, the better.
Why do people think that it is the safest way to store data on external drives?
There is some truth here. HDDs fail because of too many accesses, especially write accesses.
An external HDD (USB, SATA, eSATA) is not so often accesses as an internal drive, integrated in the computer.All these and many more topics are in the free eBook "Improve your security" available here: www.improve-your-security.org.