Quick question concerned with external HD space

Question

just a quick question
if TM only backs up files that have changed since the last backup
why does every time it back up i end up with a folder in my external HD (that acts as a time machine) that is the same size of the hard drive?
(e.g im using up 70 gigs on my macbooks HD..every time i back up with TM i end up with 70 gigs less on the external..should i just lose the space of the changed folders? :S)

Answer 1

That's the "magic" (or "smoke and mirrors") of Time Machine.
It makes "hard links" (sort of like multiple aliases) to the backups of the items that didn't change. That makes each backup, in effect, a full one.
That's how when you go into the "Star Wars" display, it can show you a snapshot of your entire system at the time of any backup.
If you look at your backups via the Finder, each dated backup folder is counting all those items that didn't change; so each one appears to be a full copy of your Mac. The sum of all the backup folders will be far greater than the amount used on the whole backup disk.
By the way, be +*extremely careful+* using the Finder on your backups -- if you change, move, or delete anything there, you can hopelessly corrupt them.
You can do everything you need to via TM.