[CentOS] Centos7 System Storage Manager: Device /dev/sdc not found

Wed Aug 2 19:37:39 UTC 2017
johan.vermeulen7 at telenet.be <johan.vermeulen7 at telenet.be>

Hello All, 

I 've gotten myself into trouble by running out of disk space. 
On an all-in-one Ovirt system some users have filled up a Windows7 virtual machine and in doing so filled up an nfs mount and in doing so 
filled up a root partition: 

B estandssysteem Grootte Gebruikt Besch Geb% Aangekoppeld op 
/dev/mapper/centos-root 50G 46G 4,8G 91% / 
devtmpfs 7,9G 0 7,9G 0% /dev 
tmpfs 7,9G 0 7,9G 0% /dev/shm 
tmpfs 7,9G 8,5M 7,9G 1% /run 
tmpfs 7,9G 0 7,9G 0% /sys/fs/cgroup 
/dev/md127 498M 106M 393M 22% /boot 
/dev/mapper/centos-home 20G 33M 20G 1% /home 
/dev/mapper/centos-var 360G 623M 360G 1% /var 
hotelcaw:/mnt/iso 50G 46G 4,8G 91% /rhev/data-center/mnt/hotelcaw:_mnt_iso 
tmpfs 1,6G 0 1,6G 0% /run/user/0 

I tried to remedy that today by using ssm to add storage. I added an extra disk witch shows up as sdc: 

[ root at hotelcaw ~]# ssm list 
---------------------------------------------------------------- 
Device Free Used Total Pool Mount point 
---------------------------------------------------------------- 
/dev/md126 208.00 MB 437.84 GB 438.05 GB centos 
/dev/sda 465.76 GB PARTITIONED 
/dev/sda1 0.00 KB 500.94 MB 501.00 MB md 
/dev/sda2 0.00 KB 438.05 GB 438.18 GB md 
/dev/sdb 465.76 GB 
/dev/sdb1 0.00 KB 500.94 MB 501.00 MB md 
/dev/sdb2 0.00 KB 438.05 GB 438.18 GB md 
/dev/sdc 465.76 GB 
---------------------------------------------------------------- 
------------------------------------------------------ 
Pool Type Devices Free Used Total 
------------------------------------------------------ 
centos lvm 1 208.00 MB 437.84 GB 438.05 GB 
------------------------------------------------------ 
------------------------------------------------------------------------------------- 
Volume Pool Volume size FS FS size Free Type Mount point 
------------------------------------------------------------------------------------- 
/dev/centos/swap centos 7.88 GB linear 
/dev/centos/root centos 50.00 GB xfs 49.98 GB 4.83 GB linear / 
/dev/centos/home centos 20.01 GB xfs 20.00 GB 20.00 GB linear /home 
/dev/centos/var centos 359.96 GB xfs 359.79 GB 359.23 GB linear /var 
/dev/md126 md 438.05 GB raid1 
/dev/md127 md 500.94 MB xfs 497.61 MB 417.29 MB raid1 /boot 
/dev/sda1 md 501.00 MB xfs 497.61 MB 417.29 MB part 
/dev/sdb1 md 501.00 MB xfs 497.61 MB 417.29 MB 
------------------------------------------------------------------------------------- 

now adding /dev/sdc to pool centos fails with: 

[root at hotelcaw ~]# ssm add -p centos /dev/sdc 
Device /dev/sdc not found (or ignored by filtering). 
Unable to add physical volume '/dev/sdc' to volume group 'centos'. 
SSM Error (2012): ERROR running command: "lvm vgextend centos /dev/sdc" 

I did a format 0 on the hard drive that makes up /dev/sdc with: 
dd if=/dev/zero of=/dev/sdc bs=8M 
I suspect I need to prepare it further before it can be added. 

Can anyone advise me on this issue? 
Many Thanks 
J.