Beta 2 has a known issue where the unplugging, disconnecting, or powering off, of a disk on the home server while backup is running can cause the drive to get stuck in a state where it cannot be re-added or removed from the system using the Server Storage tab.
Suggestion:
Avoid unplugging or powering down server disks while backup is running. Ensure client Computer backups are all complete before removing a disk.
Workaround:
If you do get in a situation where a disk is in a flakey state due to removal during backups, rebooting the server will fix the problem.