Time Machine back up?

It won't make multiple copies of unchanged files. If you're looking at your backup, you will see what looks like your entire HD in each backup folder. That's just how TM works - they look and act like the actual file(s) - but they are, for the most part, hard-links to the originals. Those take up the minimum file size of 4KB. Do a search here or Google for a more in-depth discussion of the inner workings of Time Machine.

If you are backing up a 250GB drive to a 250GB TM drive, I would say the drive is too small. The general rule-of-thumb is to have a drive 2x the size of your source drive - or, at least 2x the amount of data you plan on keeping on the drive. Anyway, it wouldn't be surprising to have a 250GB drive run out of space backing up 175GB of data. How long it took, of course, depends on your usage. It could happen over a period of many month. For instance: if you modify any metadata for your music files, they'll get backed up; System updates will cause fairly large backups, since they touch so many files; if you don't exclude virtual machine drives, they will get backed up in their entirety whenever they're used, since they're a single file; etc.

Hope this info helps.

 

One weakness of Time Machine is that if you change ANY PART of a file, no matter how large the file and small the part, it will make another complete copy. I recently renamed about 30 gb worth of videos from .mp4 to .m4v, so it copied every single video over again.

 

Of course it does, it's a new file for the filesystem man...

 

But it would be nice if it did it at block-level, only copying the bytes that have been changed. Ars Technica’s review of Leopard when it first came out went into this. If OS X was to use ZFS, it would be possible. That would mean usually much smaller and quicker Time Machine backups. I believe DropBox does this too, only updating the parts of a file that change, not the whole file.