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That's not how it works - it would be inefficient to the point of uselessness to rewrite the entire file constantly, and my 100GB TrueCrypt volume is fast to save to.

If that were the case, then terabyte-scale volumes would be prohibitively useless - and I know plenty of people with media volumes in TC.

But aren't TrueCrypt volumes completely changed when you add/remove something?

No, the crypto in TrueCrypt doesn't amplify the number of changed blocks. But it does disperse the changed blocks throughout the encrypted image, and this does result in higher bandwidth used during the sync (at least with DropBox). Here's an analysis I did a few years ago:

https://news.ycombinator.com/item?id=2546626