Now it's ~55 GB uncompressed and as you said that's without images, audio, and more.
There's a pretty big hurdle to comprehension even without compression because we are thinking in human terms. Imagine all of the prerequisite human knowledge you overlook to even approach the concept of an encyclopedic article describing something. Aliens might share knowledge by hitting each other with telepathic darts for all we know, having a completely different understanding and implementation of information, and words might require years of study on their part to comprehend. Even the golden record carries a lot of assumptions. What we know about the universe is not necessarily final, even with rudimentary things like information theory.
In the end it's a bunch of bytes, numbers really, on a disc. What are numbers, even? What if they have a totally different non-numeric system to quantify and explain their existence?
Think about finding an extraterrestrial storage device like this from our perspective. I'd safely predict 20 years before we even extract one byte of data, and a lot of that time would be arguing over it, probably. Although thinking about an alien Nobel ceremony for cracking the "extraterrestrial ceramic Wikipedia" is a pretty amusing thought.
Its about humanity trying to reverse engineer a message from space that has very few bits. One of the morals is that humans would be able to decode crazy encodings provided enough time. And more data helps a lot. With 20 GB, even compressed, common patterns could quickly be found.
I don't believe for a moment a race advanced enough to recover the disc wouldn't understand it.
http://rosettaproject.org/disk/concept/
Of course then you need to work out what the language is actually saying, but plain "glyph retrieval" can be done with a desktop microscope and some time. An alien that operates in a roughly human way (has eyes, language, linear writing) would probably understand that it encodes meaning, even if it ends up like Linear A and undecipherable.
Once you introduce a few words, the rest may be decipherable from context, especially with such a large corpus. E.g. certain words will cluster together often, and once you know one, you can guess at the others, which lets you guess at others, and so on.
Let's suppose the aliens already did this, but instead of a small disc, they wanted to send a message that no intelligent being could miss. Over a short span of 150,000 years, they redirected a bunch meteors into the moon in a pattern that encodes the last few digits of pi in a simplified resonant-fractal numbering system, which proves they know the angular momentum of the universe with fair precision. Clear and convincing evidence that would be visible throughout the solar system.
So yeah, I think no matter what we do to try to communicate with an alien intelligence, they would have to be very much like us (probably our own descendants or - who knows - ancestors) to even recognize the presence of the simplest message, much less decode it.
There is no last digit of pi.
I'm only half being sarcastic, as I've gotten lost in some of those articles trying to figure out who someone is... while I'm not sure of the encyclopedic value of those articles, they definitely have some entertainment value. For that matter, it may be worth clarifying fictional characters from those that are historic, or based on historic events.
One concept that always got me is the premise of an alien culture without sarcasm. They would have a very hard time with human culture/history.
Mexican-American war had twenty thousand words of discussion over what dash to use, and that knowledge isn't transferable even to other dashes used in title on WP.
Here's 15,000 words on dashes. The result? No concensus.
I'm waiting for the meme that conclusively starts a war. The assassination of Archduke Franz Ferdinand in 72 point Impact. That'll be a fun one for future history books to explain.
Any civilization capable of retrieving the disk should understand enough about information theory to undo compression. Instructions can also be explained in detail with pictures at the beginning, uncompressed.