[NNolan]

So, I've downloaded a few new books using the USB option. If I specify that the books are going to my old K3 then I get AZW3, but if I tell it I'm using my Oasis sometimes I get AZW3 and sometimes I don't. So, from now on tell Ammy it's for my K3 for USB downloads. I should be able to do that even if the K3 really and truly dies, as long as I don't deregister it.

While I was messing around with Calibre today I decided to convert some old epubs that I haven't read yet into AZW3 format and put them on my Oasis 2, which worked for the first 3 books I selected. Sometime between moving that first set of books to my Kindle and converting another set, Calibre lost the ability to write books to my Oasis 2, and I don't know why. I tried upgrading to the latest version of Calibre and that had no effect.
So, now I'm writing them to another folder after converting them, and moving them over via the operating system. I'm also giving serious side eye to the one USB cable that's been a common element in all my Kindle woes over the last two days. I'm going to dig up another cable and see if it makes a difference, although I'm not getting my hopes up.

Edited to add: as I suspected, changing USB cords didn't help. Bummer....