you have to copy everything on a TM drive and it sees what was changed on a completely different drive and changes the file that matches on the TM drive?
Each time TM takes a snapshot, it copies over everything that's new (and isn't excluded).
The operative word here is
new. New since when?
The answer is, new since the last good snapshot. In the beginning, when it hasn't yet made any snapshots,
everything is new.
Normally, it'll try to make a new snapshot every hour, but what if it can't? For example, you might have disconnected the backup drive. If it can't take a snapshot, it doesn't, but the next time it can it looks on the backup volume to see when the last good snapshot was, and backs up everything new since then.
