Can't erase Thunderbolt Duo 8TB on El Capitan

Used my 8TB Thunderbolt WD on Mac, works great. Now want to partition it, but need to erase first (right?) Get error message that it can not erase succesfully -“invalid name”.
Any suggestions please.

You will have to use WD Drive Utilities instead of your Mac’s built-in Disk Utility in order to change the RAID level.