https://www.mobileread.com/forums/sh...ostcount=43531
Update: Got around 15k books on the SD card. It's a 512*GB SanDisk, and I have ~342*GB left, give or take. I had a little error with Calibre having a fit over a folder, but I figured it out by reformatting as internal storage, then restarting the whole mess. Now Moon+ is scanning the books folder to import the books into its DB, so I can set up collections and then do a backup.
I say books, but it's my Instapaper bundles, magazines, fanfic, books I bought from Amazon/Google Play, and books from Standard Ebooks/Feedbooks' Public Domain catalog.
I had to take a break in the middle for a nap, as I'm recovering from surgery.