Using time machine with only an iMac's internal Hard-rive?

Yes, you can partition

 

This defeats part of the purpose of a backup, though, which is to protect against hardware failure of a main drive.

 

While this is absolutely true, the OP's idea would be great for notebook users: immediate version backup on the local drive while away from the main external back up drive.

I plan on giving a few GB's (by way of a partition on the local drive) to Time Machine and then have another external drive for the "real" Time Machine back up.

 

Is the space directly proportional to that data being backed-up or is there some compression?

Suppose I have 70GB Music - 110GB Pics/Vids - 20GB Applications

Will I require exactly a 200GB HDD?

 

A partition on the same time is good for a backup OS for the times you ruin the startup OS with an install, or the times when the disk catalog gets thrashed.

But these days the drive is more likely to go terminal before suffering a corrupted disk catalog.

 

Not really. First, I was applying it more to the specific case of an iMac - its not very portable. Even if it was a portable machine, it isn't possible to have two Time Machine backups running, one on the internal and one on an external. If you're looking for something to keep backups with a granularity of less than an hour (say, automatically when changes are made) that is portable, Time Machine (sadly) isn't the right tool. Hopefully some improvements will be made in the future to make this possible for laptop users like myself.

 

Part of the bonus of having an external hard drive is if your Mac hard drive failed for whatever reason, or your Mac got stolen, you'd still have all your data backed up.

 

DiskStudio

I used DiskStudio to split my 1Tb disk in my AL-iMac in half... and it did not disturb my installation. When it was done the 1Tb drive was split in two; one called Macintosh HD and one called Macintosh HD_2. Made me very as I thought it would wipe the installed partition clean

 

Gosh darn it. I'm moving to Vista.

 

Don't you dare utter those terrible words here! all the way!

 

Ok. I'll stick around until 10.6, but I'm not guaranteeing anything. Because of the way Time Machine works, I don't see why using it on two drives is impossible. Maybe someone else could set me straight?