This article describes the cause and possible solution for the backup error:
ERROR: homePool/home/agents/[agentName] is not mounted, cannot proceed with backup
The ZFS file system's storage volume has become unmounted on the Datto device.
Most often, this occurs if the device experienced an unexpected shutdown during a restore or device operation (File Restore, Screenshot, Running Virtual Machine, Bare Metal Restore).
A quick and effective solution is to simply reboot the device.
On larger Datto devices, it will generally take up to half an hour for the storage pool to remount.
If the error does persist after a reboot, please contact Datto Technical Support so that a technical support expert may remotely access the device and determine why the storage pool is failing to remount automatically.