When I used VBoxManage in host OS to check the status, it showed that the hard drive was 'captured'.
And then in guest OS, I checked the menu in Virtual Box : Device->USB, and saw that my Seagate hard drive was indeed 'checked'. I think this is actually 'fine', since USB hard disk can only captured by one OS. When I inserted USB drive, to begin with, the host OS Mac showed that 'hard disk drives were not ejected normally'. I enabled all ports in my host OS, and created two filter for USB devices: one is empty, the other is Seagate. To begin with, I list my environment information as following:
