I just got myself a 1 TB portable hard drive eek and so I'm finally backing up using Time Machine. It's great - I like the combination of beautiful Mac aesthetics and space-saving functionality. The problem is that I have a particularly large file which changes a lot - that is, my 15 GB Parallels virtual machine. So every time this has even a minor change, it looks like Time Machine backs up a new copy, which will start to fill up my 400 GB Time Machine partition fairly quickly. Is there any way to set this to either only be backed up, e.g. every week or month, or to have it never backed up except when I manually do so?