I tried the steps and it didn't work: kobo ignored my uSD Card ebooks collection as if it wasn't there.
After a factory reset, I'm now on firmware 3.19.5613 and I'm (patiently...) reloading my uSD card from scratch using calibre.
Firmware 3.19.5613 has none of the page refresh bugs of 3.12.1 and it's actually faster on the Collections manager.
Unless something signficant enough happens with any of the 4.XX.YYYY firmware versions, I'm staying with 3.19.5613 for the forseable future.
Thanks for your patience and advice,