Your disk was unmounted by ntfs-3g:
Apr 13 18:16:16 Medserver unassigned.devices: Adding partition 'sda1'...
Apr 13 18:16:16 Medserver unassigned.devices: Mounting partition 'sda1' at mountpoint '/mnt/disks/flashbackup'...
Apr 13 18:16:16 Medserver unassigned.devices: Mount drive command: /sbin/mount -t 'ntfs' -o rw,noatime,nodiratime,nodev,nosuid,nls=utf8,umask=000 '/dev/sda1' '/mnt/disks/flashbackup'
Apr 13 18:16:17 Medserver ntfs-3g[24979]: Version 2017.3.23 integrated FUSE 27
Apr 13 18:16:17 Medserver ntfs-3g[24979]: Mounted /dev/sda1 (Read-Write, label "", NTFS 3.1)
Apr 13 18:16:17 Medserver ntfs-3g[24979]: Cmdline options: rw,noatime,nodiratime,nodev,nosuid,nls=utf8,umask=000
Apr 13 18:16:17 Medserver ntfs-3g[24979]: Mount options: rw,nodiratime,nodev,nosuid,nls=utf8,allow_other,nonempty,noatime,default_permissions,fsname=/dev/sda1,blkdev,blksize=4096
Apr 13 18:16:17 Medserver ntfs-3g[24979]: Global ownership and permissions enforced, configuration type 1
Apr 13 18:16:17 Medserver ntfs-3g[24979]: Unmounting /dev/sda1 ()
Put the disk in a PC and see if there are any issues with the disk. Do a file system check. I see nothing in the log that indicates why it unmounted.
You'll need to reboot before you put the disk back in the server. UD still thinks the disk is mounted.