While I comment on my weekly backup method, I don't bother backing up my Data drives, they are backed up as they are created, I rarely have less than four copies of any data.
My "important" setup data, e-mail and browser favorites are backed up daily to a pair of thumb drives that are mounted internally in my desktop.
I replace one of the pair every six months.
Yeah, I know, they are limited on the number of writes before the drive dies.
Well... I erase and discard them after a year of use.
and at $7 each it costs me $4 every six months.