I got some guidance today on the classiccmp mailing list, and finally figured out how the AED 2200 Disk Controller and the Diablo 30 connect to the Unibus. So, with that fresh in mind, tonight I decided to de-rack the Diablo 30 and inspect it.
It came out easily enough, and I wiped it down with a moist cloth. It was startlingly filthy, but after a few passes with the cloth, it started to look like a disk drive again. And with the dust and dirt off the front panel, I was finally able to see inside. Yes, there's a disk pack in there. I don't know how bad that is - I'm pretty sure you're not supposed to move these drives with a disk pack inside, but I'm not sure. The head was obviously locked (there's a big "LOCKED" tab that is very clearly visible), so maybe it will be OK?
The screws that hold the top cover to the chassis were all badly rusted, but still came loose easily. Once inside I saw that things are mostly good looking. There's rust on some of the parts, but once again the PCBs and electronics look good, and the disk head looks OK. For now, I just buttoned everything back up and left it de-racked. I will get around to further inspection when I get to the point of trying to actually get it powered on. That will be a while.