I've just upgraded from Lollipop (5.1.1) to Marshmallow (6.0) on my Motorola Moto G 3rd Gen.
I found that both my installed reading apps, CoolReader and Moon+ Reader, were unable to access the microSD card from within the app. As the problem appears to be widespread I'm assuming other apps have the same problem.
It appears this is because of changes made to the file path used by Marshmallow for the micoSD card and appears to affect many devices from my research trying to find out why this was happening and for finding a solution.
It is possible to open Epubs from the microSD card using Moon+ Reader and CoolReader GL, but it can only be done by locating the book in a file manager and then selecting the Open option. A very longwinded way of opening a book if you read on an Android device regularly.
I have found that the
Pocketbook app (
https://play.google.com/store/apps/d...y.reader&hl=en) can locate the microSD card and open books directly.
To find your microSD card, open the Menu and select 'Folders'. You need to find a folder called 'Storage' and open it. In there you will find a folder with a name something like 1234-5678 (the name appears to be different for each card). This is your microSD card. Now you can navigate as usual.