G-drive cannot make back ups sinds installation of Mojave MacOS

Since I have updated MacOS with Mojave no more back ups are made on the G-drive. A back up is being prepared by Time Machine but is not executed. Ik have installed the latest firmware on the Mac but this doesn 't change anything. However, I still can use de G-drive which is USB connected for a back up of selected files. Has anyone experienced the same problem and knows how to solve it?