I bought a few months ago, an external hard drive, which is something I had meant to do for ages but was also one of those things that you never really seem to get around to doing. It is one of the best things I have bought for my imac as I can copy stuff across to back it up and it is in my view a better alternative to the rather crummy mac "backup" application which you can only use to copy a limited amount of stuff to a burn disk.
It’s especially useful for backing up sd cards and for swopping stuff across between them. I am always doing this when I change devices.
They are pretty cheap…I got an 80gb one for less than £80.