If Android device is connected via USB cable, but "Mount disk now" button is grayed out, try to do the following:
1. Disable "Disk drive" mode on your device:
Settings –> Connect to PC –> Default connection type –> Disk drive
2. If "Mount disk now" button in "Mount disk" plugin is still grayed out, reconnect the device via USB cable (unplug and plug in again).
This will help SyncMate detect the device’s storage space and mount it as additional Mac disk.