I am also seeing this problem with my tablet and have for quite a while.
calibre: 2.51 OS: Mac 10.11.3 (El Capitan) Device: Samsung Galaxy Tab S - Android 5.0.2
I can access my tablet when plugged in using the Android File Transfer app.
FWIW, I don't have this problem when using my phone (Samsung Galaxy S4 - Android 5.0.1).
I am also seeing this problem with my tablet and have for quite a while.
calibre: 2.51
OS: Mac 10.11.3 (El Capitan)
Device: Samsung Galaxy Tab S - Android 5.0.2
I can access my tablet when plugged in using the Android File Transfer app.
FWIW, I don't have this problem when using my phone (Samsung Galaxy S4 - Android 5.0.1).