I think I made the mistake of disconnecting it premeturely last time I was trying to sync it.
Long story short, I have two user accounts on the same computer.
When my Kindle was plugged in and syncing with the Calibre on my main user account, I switched users without ejecting or logging off.
After switching users, and starting Calibre (I have two user accounts so that I can keep working on another library while Calibre pluggs away dealing with some time-extensive job that otherwise would have me twiddling my thumbs)
When Calibre started up on the second user, I forgot that the Kindle was still plugged in and likely running on another library. I yanked out the usb cord and did a very bad disconnect.
When I returned back to the main library on my main user account (after logging off the second account). This was when I had problems.
What would happen is that Calibre would load the booklist from the Kindle, but at about 60% it would get that error I posted.
I updated the firmware, but still kept getting that error.
So...I figured on trying to load it up on that second account. When I attached the Kindle, I got that "scan and fix" option, so I did.
Once that was done, Calibre was able to get the booklist and send the metadata to the device. Up until that point the metadata file on the device was 0 bytes.
So, I suspect this was a very user-specific problem, and I learned my lesson
The books read fine on the device. It was the getting of the booklist that kept erroring out at 60%