RCA maven pro Android tablet not totally recognizing USB drive
Marshmallow requires apps to have permission to access files. If Memory Zone did not request access permission when you first opened it then it was not written for Android 6.0. Try this: In settings open the apps menu. Tap the gear icon in the upper right corner and then select “App permissions”. Scroll down the list and tap “Storage”. Now find your Memory Zone app in that list and flip the switch on the right hand side. 

If that doesn’t work, I would go with a file manager that knows about Marshmallow’s permissions. Personally, I use Solid Explorer and love it because I can have the USB in one pane and Internal Storage or SD card in the other.