El Capitan doesn't recognize internal HD
I've got a Mac Pro 2009 running 10.6.8 from a SSD drive.
I installed a WD 8 TB red HD. All perfect.
In a 2nd partition of the SSD drive, I've installed El Capitan.
But El Capitan doesn't recognize the WD HD. If I boot 10.6.8 it's Ok. But if I boot El Capitan, the HD is missing, even in Disk Utility.
Any ideas?
Thanks.
Mac Pro, OS X El Capitan (10.11.4)