update....I ordered and received from bhphoto this external drive for my iMac:
G-Technology 3TB G-Drive with Thunderbolt
this was so easy to set up and use....about 30 minutes after unboxing i had already backed up nearly 1/2 a TB of photos/videos on it with a simple drag and drop.
for a non tech savvy guy, this was nice.
sleek and solid feeling too.
OK, maybe I am a little late to the party, but your back-up drive being full is not a problem with time machine. If you have 500GB on your main drive, and your 1TB back-up is almost full, it means your back-up drive has 500GB of current stuff, and almost 500GB of old deleted stuff, and old versions of current files. The old deleted stuff can be stuff that you deleted like documents, tax recored etc. (but you have to have deleted them). It can be stuff that you updated, like old copies of a resume, photo etc. (but not the current copy no matter how old) And it also includes stuff like cashed web pages, system preferences, and old versions of files that you have updated.
My recommendation is to have more than one back-up drive, Western Digital or otherwise, and keep one at work/grandma's house. Swap them 1 eek or so. That way if robbed, or the house burns down, all is not lost.
Alternative to swapping, You might also consider an online off-site storage system like carbonate. Does not need to be the whole 500GB, just the OMG can't loose that stuff.
Tex...yes, I'm going to sign up for crashplan for my off site...good suggestion.
So how do I delete some of the "extra stuff" that time machine has saved?