In my case, problems began with the automatic update to fw. 2.6.1 by Kobo. Later, a sync with the Kobo Desktop brought me back to 2.5.2, but the difficulties with the uSD card remained.
At present, the reader sort of calmed down; problems appear not so frequently.
My model is N905 (the older one), and I don't know if it's recommendable to update this one to 2.8.1. Anyway, as long as there are not so many

problems, I prefer avoiding any changes that could cause still more trouble.