[quantumcat76]

I haven't tried using an SD card with less books (yet).
And normally I'd say, YES, I have been gradually adding.
But I this is the second factory reset I did.

The first one, took me about an hour or so -I don't know ...a long time...- to load the same 16Gb card with the same books.

Point is: it worked.
But, now, after the second factory reset, I get stuck at 94% after 8 hours or so ("overnight").

But I will try to add an SD with less books.