Could someone share a link to the original presentation? I'd love more technical details.
Is this SDHC or SDXC compatible? If they support SDXC, how do they reconcile "fully open source" with the fact that SDXC requires exFAT which is proprietary and protected by patents?
Building something like this requires an SD Slave Controller. It would be absolutely fantastic if they open source that! Fully verified and SDXC compatible? This makes me drool. Unfortunately the necessary information to implement and verify is walled behind NDAs (SD Association), unless of course you have reversed it with a logic analyzer. I don't think that's what Google has done, which makes me very curious.
If the slave controller is not open source, that begs the question of how data is transferred to the RTOS. A closed source core with DMA to whatever you run on it will exclude it from certain privacy-sensitive use cases.
If anyone at Google is reading this, consider using cores from cryptech.is for your crypto. You're already a funder so you might as well reuse them.
If anyone is interested in what I just said and you think you can contribute I would love to talk with you. Email me at stromberg@mullvad.net.