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. Solid state drives are an improvement but even these will fail one day. There's a finite limit to the number of times they can be 'written to'.

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 DVD or a Blu-ray disk.

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

Mirror Drive
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.

NAS Storage
A 'NAS Box' is a large capacity external drive. This can back-up all data / image files and capture 'An Image', that is to say a complete record of your system, programs, everything. Should the main drive fail this will greatly speed up recovery time. More elaborate, yes, but it all depends upon the level of security and feeling of safety you require.

NAS by the way is an acronym for Network Attached Storage.

Ghost Drive
Although less popular at present, this is an alternative to a 'NAS Box'. Like NAS, the 'Ghost' will copy / back-up everything on the system including all programs and associated data.

Internet Back-up option
One other option is to back-up using an on-line storage and retrieval system. There are many available.