I'm sorry that getting a reply to your question was this unbearably slow, but I didn't know how to answer your question and I was REALLY hoping a system moderator would come over and help you. (Not that they couldn't, they're probably just really busy.) I'll do my best, but I am NOT a Linux Guru, I've just played around with computers for a while. So my information may or may not be correct.
You have to understand that Linux does not treat permissions like Windows (or to a lesser extent, Mac). What's probably happening is that you don't have the authority to access another OS: it sees the second hard drive as its own independent computer. It doesn't know that it's really part of the same computer. You running Gparted is irrelevant, it just allows you to partition drives while the desktop is up. It doesn't "link up" the two drives together. So, really, you have no slave drive at all, just two separate independent drives.
What I would recommend is kind of the lazy man's out, but is SHOULD still work. I would only install Ubuntu on only one of the drives. (This will be your master drive), then only install an ext4 filesystem on the other WITH NO OS!
That way, your makeshift "master drive" can still operate as normal, and you can move files around on the other hard drive. Please let me know if this helps, I'd be glad to do what I can.