Additional storage devices mounted with unassigned devices plugin are being counted toward attached storage devices licensing.
For example, if you have a basic license for up to 6 attached storage devices.
And you have 6 external USB devices attached and mounted via the unassigned devices plugin, you can't start your array because too many drives are present.
Unraid's license terms, article 2 states "Additional storage devices can still be utilized directly with other Unraid features such as Virtual Machines or the unassigned devices plugin."
Is this by design or a bug?
Workaround
Detach the external USB drives, reboot, start the array, reattach drives and mount.