The disk was not unmounted cleanly:
Aug 26 18:03:03 Tower unassigned.devices: Adding disk '/dev/sdf1'...
Aug 26 18:03:03 Tower unassigned.devices: Mount drive command: /sbin/mount -t 'exfat' -o rw,auto,async,noatime,nodiratime,nodev,nosuid,umask=000 '/dev/sdf1' '/mnt/disks/External_HD_Drive'
Aug 26 18:03:04 Tower mount.exfat: volume was not unmounted cleanly
Aug 26 18:03:04 Tower unassigned.devices: Mount of '/dev/sdf1' failed: 'FUSE exfat 1.3.0 WARN: volume was not unmounted cleanly. fuse: mountpoint is not empty fuse: if you are sure this is safe, use the 'nonempty' mount option '
Aug 26 18:03:04 Tower unassigned.devices: Partition 'WD Elements_25A3' cannot be mounted.
Click on the check mark next to the mount point and see if it can be fixed.
You also have a UPS communications issue:
Aug 26 17:06:39 Tower apcupsd[5597]: Communications with UPS lost.
Aug 26 17:15:51 Tower emhttpd: spinning down /dev/sda
Aug 26 17:16:45 Tower apcupsd[5597]: Communications with UPS lost.
Aug 26 17:16:56 Tower emhttpd: spinning down /dev/sde
Aug 26 17:17:22 Tower emhttpd: spinning down /dev/sdd
Aug 26 17:17:22 Tower emhttpd: spinning down /dev/sdc
Aug 26 17:26:51 Tower apcupsd[5597]: Communications with UPS lost.
Aug 26 17:30:52 Tower emhttpd: spinning down /dev/sda
Aug 26 17:36:57 Tower apcupsd[5597]: Communications with UPS lost.
Aug 26 17:45:53 Tower emhttpd: spinning down /dev/sda
Aug 26 17:47:03 Tower apcupsd[5597]: Communications with UPS lost.
Aug 26 17:51:07 Tower emhttpd: read SMART /dev/sde
Aug 26 17:57:09 Tower apcupsd[5597]: Communications with UPS lost.
You need to get that fixed.