Error: concurrent.futures.process._RemoteTraceback: 
"""
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/middlewared/plugins/zfs.py", line 148, in detach
    target.detach()
  File "libzfs.pyx", line 376, in libzfs.ZFS.__exit__
  File "/usr/local/lib/python3.6/site-packages/middlewared/plugins/zfs.py", line 148, in detach
    target.detach()
  File "libzfs.pyx", line 1343, in libzfs.ZFSVdev.detach
libzfs.ZFSException: Can detach disks from mirrors and spares only

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.6/concurrent/futures/process.py", line 175, in _process_worker
    r = call_item.fn(*call_item.args, **call_item.kwargs)
  File "/usr/local/lib/python3.6/site-packages/middlewared/worker.py", line 133, in main_worker
    res = loop.run_until_complete(coro)
  File "/usr/local/lib/python3.6/asyncio/base_events.py", line 484, in run_until_complete
    return future.result()
  File "/usr/local/lib/python3.6/site-packages/middlewared/worker.py", line 93, in _run
    return await self._call(name, serviceobj, methodobj, params=args, job=job)
  File "/usr/local/lib/python3.6/site-packages/middlewared/worker.py", line 85, in _call
    return methodobj(*params)
  File "/usr/local/lib/python3.6/site-packages/middlewared/worker.py", line 85, in _call
    return methodobj(*params)
  File "/usr/local/lib/python3.6/site-packages/middlewared/schema.py", line 668, in nf
    return f(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/middlewared/plugins/zfs.py", line 150, in detach
    raise CallError(str(e), e.code)
middlewared.service_exception.CallError: [EUNKNOWN] Can detach disks from mirrors and spares only
"""

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 165, in call_method
    result = await self.middleware.call_method(self, message)
  File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 1096, in call_method
    return await self._call(message['method'], serviceobj, methodobj, params, app=app, io_thread=False)
  File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 1044, in _call
    return await methodobj(*args)
  File "/usr/local/lib/python3.6/site-packages/middlewared/schema.py", line 664, in nf
    return await f(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/middlewared/plugins/boot.py", line 209, in detach
    await self.middleware.call('zfs.pool.detach', 'freenas-boot', dev)
  File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 1100, in call
    return await self._call(name, serviceobj, methodobj, params, app=app, pipes=pipes, io_thread=True)
  File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 1041, in _call
    return await self._call_worker(name, *args)
  File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 1061, in _call_worker
    return await self.run_in_proc(main_worker, name, args, job)
  File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 995, in run_in_proc
    return await self.run_in_executor(self.__procpool, method, *args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/middlewared/main.py", line 971, in run_in_executor
    return await loop.run_in_executor(pool, functools.partial(method, *args, **kwargs))
middlewared.service_exception.CallError: [EUNKNOWN] Can detach disks from mirrors and spares only