I remember back in 2007-2010 I gave a lot of thought about how to share data securely between my Mac (I was just getting into Mac back in 2007, around when they switched to Intel) and my "legacy" Linux machines (I ended up dumping Mac in 2011, so keeping Linux in the loop turned out to be a good idea). Inevitably, TrueCrypt came up because there are simply so few cross-platform on-the-fly encryption systems. But after 2014's very strange shutdown of the TrueCrypt project, as well as the strange licensing situation around TrueCrypt, I expected to see a bunch of alternatives emerge (sort of like happened with RSS after Google shut down Google Reader). But it doesn't seem like this happened. Now in 2019, when I search for cross-platform disk encryption, VeraCrypt comes up, which is essentially a continuation of TrueCrypt, and everything else is either barely maintained, only for one platform, or closed source. This surprises me somewhat, but given how much work goes into creating an maintaining advanced security software, perhaps I shouldn't be!

VeraCrypt - Free Open source disk encryption with strong security for the Paranoid