The sdt disk is marked as pass through. UD will not mount a passed through disk.
Feb 14 16:04:26 Tower kernel: sd 12:0:0:0: [sdt] Attached SCSI disk
Feb 14 16:04:27 Tower unassigned.devices: Disk with serial 'nal_USB_3.0_2015033100081', mountpoint 'TO_Exter_nal_USB_3.0' is set as pass through.
Feb 14 16:36:21 Tower unassigned.devices: Error: shell_exec(/usr/sbin/hdparm -C /dev/sdt 2>/dev/null | /bin/grep -c standby) took longer than 10s!
The timeout error is UD trying to get the disk standby status. That's probably a USB cable or port issue, or an issue if the disk is in an external bay.