RAID setup not "safe"

Hello all,

I have been saving my photos to 2 500gb drives which mirror each other (RAID1).

I was led to believe that if one drive fails, I just replace it and all is fine, however a post on another thread led me to believe that this is not the case.

Am I wrongly concerned?

(I do additional backups of the RAID drives as well on other external drives on about a monthly basis.)