For me it means:

1. Multiple copies. i.e. I have a set of backup media I rotate among. So that when a virus/other pest is introduced or other error I can go back in time. Also, there are copies off site while backup is occuring so that when a disaster strikes, the data survives.

2. Write protection. Tapes, CDR, DVDR and such have this. The hard disk systems I see don't have this. Connect a hard disk to an infected system and it is now infected.

There are more criteria, but it's NOT UP TO ME to decide if you feel safe with your system. You must live with its cost and what happens if a bad thing happens.