I have a storage space set up on Windows 8.1 of 4 hard drives initially.  They consist of two 8 TB drives, a 6 TB drive, and a 4 TB drive.  This initially gave me 26 TB which really turns out to be around 23 TB.  Then I set it up as a 2 way mirror pool.  Which made my maximum space 11.7 TB.  Then, I added another sata port multiplier and 4 more hard drives consisting of three 6 TB drives and one 3 TB drive.  I added it to the storage pool space and all 4 of the new drives along with the 4 of the initial drives in the storage space show up when I list the drives in the pool.  

Now comes the problem.  When I try to change the maximum storage space from 11.7 to say 20 TB  it says that the new storage space is not valid.  Then, if I try to change the maximum storage space to even lower than 11.7 it says the new storage space is not valid.  Any value I try other than 11.7 TB it errors out saying the new space is not valid.  So, I can't change the maximum storage space.

Any ideas or suggestions would be greatly appreciated.

Thank You

Hi,

Based on my test, I reproduced this problem in my testing machine. After add another disk to storage pool, the storage space size couldn't be change casually. 

In my opinion, this problem should be related with the new added drive. Maybe you can try to remove the later drive and change its size for test.

You can try to use Remove-PhysicalDisk command for test.

Refer to the link below for more details:

https://technet.microsoft.com/en-us/library/hh848697%28v=wps.630%29.aspx?f=255&MSPPError=-2147217396