In fact a well as -rw and +rw there is also -r and +r
The minus and plus denote to slightly different technologies though to be honest they are very similar and most devices can cope with both. I tend to use minus for computer work but it is up to you.
The r and rw denote different things.
An "r" disk, once written on, cant be deleted or overwrittten.
An "rw" disk CAN have stuff deleted off it.
So for backups use R disks not RW.
While an external hard disk is a very useful thing to have nowadays they can crash so I would back photographs up to DVD as well just to be sure.
Better to have too many backups than not enough.