Each nandroid backup is a seperate backup. It does not replace the last backup. Depending on which recovery your using, it should be in a folder in your sdcard. Clockwork recovery has a folder named clockwork for your nandroids. Amon-Recovery has a folder named nandroid. Once you do a nandroid, you can change the name of your backup, remember NOT to put any spaces in the name. Keep it simple, for example... Myns22OCed Or CM610

Hope this helps.

 

If your just updating a backup of the same rom, you can delete the older ones. I have 1 backup of each rom I use. Also good to copy your nandroids to your computer incase your sdcard gets corrupted or fails.

 

I rename mine so I know what's up

 

I have had much better success creating a nandroid from the recovery menu than just creating a backup via ROM Manager. Either way, I rename them as well. 1 Stock nandroid, 1 Myn nandroid. I just deleted my Fresh nandroid because I NEVER used it again.