I have the Time Machine software on my system. My friend told me that it always occupies more space by downloading all the files every time from the system it starts taking back up.
I use a back up for two reasons: to recover occasionally a file that's been garbled somehow, and, by far primarily, to rescue me if my main hard drive fails. If that drive fails, everything you have on your computer is gone. Period.
"Rescue me" means, get me back in operation as close as possible to where I was before the drive crashed, as fast as possible.
If the main drive fails, you want to be able to have your backup drive become your main drive until you get a new main drive installed, which could take several days. That means your backup hard drive should be capable of two things: starting your computer, and copying all the data -- system files as well as your files -- back to a new drive when it's replaced the old one.
Time Machine's files are stored in a special format that will NOT allow it to restart your computer, but it will allow you to copy all your files to the new drive... IF you can get your computer running again first. So it does one of the two things I require from a backup.
Notice that the amount of space backup software, or Time Machine in particular, requires to record things has not once come into my comments.
For my backups, I use a commercial program called SuperDuper, but the shareware Carbon Copy Cloner will also work fine; I have used both, and they are both excellent.
I set them to back up as often as I require with what I do, which is daily, in the middle of the night. I use as a backup drive one that is the same size as my main drive and it must have a Firewire connection because a drive that uses a cheaper USB connection won't restart an Apple computer - only a Firewire drive can do that.
I tell the software to make a clone of my hard drive. They then copy every bit on it to the backup drive (it only takes a couple of hours the first time, after that their techniques reduce the backup time to about 20 minutes).
Once the software does this kind of a backup, the uneasy feeling in the pit of my stomach goes away.
/ Peter Morgan