External Volume not accessible on other hardware

I'm using an external 2TB HD together with my NAS540 to backup some old data. Now I'd like to connect that external volume to read data from it directly to another PC as the transfer speeds are way higher over cable than network.
However when I connect the drive to a mac or a pc, the drive is recognised but it can't be mounted. 

Is there a way to read data from this external disk?


#NAS_Dec_2019