Someone Somewhere :
The other issue with compression is if you're using any form of encryption other than in the drive - theoretically, it's indistinguishable from random data. Which does not compress well.
This is very true. As a general rule, one should compress first, and encrypt later. I can't think of an easy way to do this to an entire drive currently, except indeed trying to find a drive that has hardware encryption. Problem is, as far as I know, all of these are external ones. Maybe if you could find an eSATA one that does this you could hack its enclosure into a hardware encryptor, but most of them are USB I think. That said, there's no reason to encrypt your OS and most of your applications, and many other data types are often already compressed. I myself run a compressed filesystem, and except for the executable files, the gains aren't that big. But of course, those are the ones you want to load quickly, so this is where compression helps the most.