The simple answer is: Back-up, Back-up, Back-up.........................

One needs to be secure. In the days of film we were content to have the negative or transparency stored somewhere safe and if it was really important, a copy negative / transparency stored somewhere too - and we took the risk.

Digital files can be deleted, corrupted, renamed, lost or stolen.

Drives Do Fail
Do remember it's not a case of 'if' a drive fails, it's 'when' a drive fails. Drives are electro-mechanical devices and be certain, they will fail.

My procedure is:
First: As soon as possible after using my camera, I copy the memory card and I verify the data before I re-format the card.

Second: Copy the data to a DVD

Third: Back up to a secondary, external drive. The external drive can be stored away from the main computer system in case of theft, damage or fire.

Fourth: Continue to back-up, including copying to DVD as work progresses and needless to say, back-up all completed work.

Mirroring
If you can't always remember to back-up to the external drive as regularly as you should, then invest in a 'mirror-drive' system. The system comprises two identical drives linked together with one drive copying to the other in a constant stream. If you make one half of the 'mirror-drive' an external drive then you have the advantage of being able to store the files off-site for increased security.

Ghost Drive
This is a more advanced option to the 'mirror drive' as a 'ghost drive' will copy / back-up everything on the system including all programs and associated data. More elaborate, yes, but it all depends upon the level of security and feeling of safety you require.

Internet Back-up option
One other option is to back-up using an on-line storage and retrieval system such as 'Carbonite'.