One way to predict the relative safety of your data is to look at MTBF for the various media that you might use. 

For example, SanDisk has announced industrial grade CF devices with an MTBF of 3 million hours -- that is 342 years.  Non-industrial grade have a MTBF of 1 million hours -- or 113 years. 

By contrast, HDD offer somewhat lower MTBF.  As an example, the Seagate Baracuda 7200.10 series has an MTBF of 700,000 hours -- 80 years. 

BTW, a high MTBF does not (can not) guarantee that YOU will never see a failure -- or that a particular device will work for as long as the MTBF predicts.  All that it can say is that on a statistical basis and considering many of those devices, that is the statistically expected average working life of the device.  ... and if the device receives Percussive Maintenance (also known as a Technical Tap), the MTBF of that device may change.  [grin]

Doing the backups to 2 different devices protects you against 1 failure ... provided that when you see the failure, you promptly obtain a replacement device and copy your data onto it. 

BTW, the WIKI article raises some questions about the use of MTBF as a reliability indicator. 

--
Martin