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How in the world does Windows know what blocks were used by files that you then deleted in OS X?

You're right. It appears I'm mistaken. I assumed it was more of an issue of cleanup, but it's pretty obvious that running the TRIM utility in Win7 is useless for my Mac partition.

Nonetheless, the question in my post was more of the point. I wish Snow Leopard had TRIM support.

Yeah, if cleaning up were as simple as that, you wouldn't even need OS support, the drive could just trim itself as it went. But it's been 20 years since anybody designed disk geometry into a filesystem, and while TRIM may not be as complicated as all that, it still requires more bookkeeping than the OS usually does.

Optimally using TRIM, such that the OS will defragment the 512K blocks online instead of reusing them, is even more work, and I'm not sure that's what Windows is doing. They're just issuing TRIM opportunistically, AFAIK.

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