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I want to get the most of my HDD, what shouldn't i back up?

Things easily replaced?

say...i may backup my purchased folder of my itunes library, but not all the manually added CD's in my collection. It may be a pain in the ass to re add them, but I do have physical copies of those.
 
Any virtual machine created by Vmware Fusion or Parallels. Just manually back this up yourself. These are typically many gigabytes in size, and they change in size/composition every time you use them. Therefore, Time Machine will back this up many, many, many times the more you use it. So, don't have Time Machine back them up. You'll fill up your drive way too quickly. Back them up yourself.
 
Parallels or VMware Windows images. Anything that is large that changes frequently but only subtly. Scratch folders or anything uber temporary. Torrents, if you're into such things.
 
Your downloads folder if its stuff like zip files, installers, and dmg files. If you can just download them again from the net why keep them?

Systems files if you also clone your drive. The only downside of this is that you can't restore the entire drive from backup but if you clone the drive too then it's not a big deal.

Pics/Video if you already have them on other media (DVD, SD Cards, etc).
 
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