That disk has errors:
Mar 10 09:07:57 unRAID unassigned.devices: Mount drive command: /sbin/mount -t vfat -o rw,auto,async,noatime,nodiratime,nodev,nosuid,iocharset=utf8,umask=000 '/dev/sdc1' '/mnt/disks/ST1000DM003-1CH162_S1DETV80-part1'
Mar 10 09:07:57 unRAID kernel: FAT-fs (sdc1): utf8 is not a recommended IO charset for FAT filesystems, filesystem will be case sensitive!
Mar 10 09:07:57 unRAID unassigned.devices: Successfully mounted '/dev/sdc1' on '/mnt/disks/ST1000DM003-1CH162_S1DETV80-part1'.
Mar 10 09:07:57 unRAID unassigned.devices: Adding disk '/dev/sdc2'...
Mar 10 09:07:57 unRAID unassigned.devices: Mount drive command: /sbin/mount -t ext4 -o rw,auto,noatime,nodiratime,async,nodev,nosuid '/dev/sdc2' '/mnt/disks/ST1000DM003-1CH162_S1DETV80-part2'
Mar 10 09:07:58 unRAID kernel: EXT4-fs (sdc2): warning: mounting fs with errors, running e2fsck is recommended
Click on the check mark when the disk is unmounted and UD will check the disk and fix the error.
Why are you using a VFAT file system for a VM? It would be better to use BTRFS, or XFS.