If my Mac HD is 1 GB, though contents just 100 GB or so, my main external for documents is 3 TB, and about 2.5 is filled up. You want your TM external to be? Thanks.

Not sure how you can have 100GB of contents on a 1GB HD.

The rule of thumb is that for Time Machine backup purposes your external drive should be at least twice as large as your HD — eg, 500GB HD > 1TB external.

Like you, I don't have a large amount of stuff on my drive (even now it's only 200GB on a 320GB drive) and I have two backup drives - SuperDuper on a 500GB drive and Time Machine on a 320 GB drive.

For the Time Machine drive, it's a reflection of how I use it. In my case, Time Machine is most useful when I'm working on a large project and want to go back to the version of a document from a few hours or days ago. However, once a project is completed and I have my final version, I don't want every " slightly different" copy of a given document.

I'm sure that someday the Time Machine will start dropping the oldest backups in order to make room for new ones, but I don't lose too much sleep over that as everything I want to keep permanently is on my Super Duper backup.

Thanks Gleber/ Rick
Well for years I have just a 800 GB TM, and I've excluded lots of non changing stuff, I have my own "main folder", on an external drive. When I open any random folder on it and then enter time machine, TM just goes to user, name folder -- and that is the Mac OS drive? Any ideas on what is going on? That folder is not excluded.

So that's why I'm thinking of just getting a huge HD as this method is maybe not working?

Griebler, If I buy a huge new one, that would take care of my 3TB, as well as the Mac OS which is on a 1 TB.

No clue. The 'rule of thumb' I provided is about as far as my expertise [sic] takes me.