Cannot resize after growing a Raid5

Hi all

I built this OMV machine a couple of months back with 4 4tb drives in a raid 5. The machine has 6 4tb drives installed and I am trying to add them one at a time to the array.
I was able to grow the array by 1 drive last night but this morning I am unable to resize it. When I click on resize and than ok I get the error below.
Array is formatted ext4.

I know there is other important info that will be needed but not sure as to what or how to get it.

Thanks

Failed to grow the filesystem '/dev/md0': resize2fs 1.42.5 (29-Jul-2012) resize2fs: Device or resource busy While checking for on-line resizing support Filesystem at /dev/md0 is mounted on /media/b21561d7-95f7-4010-811e-5f1da55e6de7; on-line resizing required old_desc_blocks = 1563, new_desc_blocks = 1863

Strangely the volume is resizing even with the error message.
Just in the time I wrote this post it went from 13.11TB to 13.13TB so it appears that it is resizing.

Is there any screen or command line to check status of resizing?