what about 6520 on El Cap, 
i've done everything that is said here : 
https://osxlatitude.com/index.php?/topic/8506-dell-latitude-inspiron-el-capitan-clover-guide/&do=findComment&comment=54632 
  
using the : 
----------------------- 
Legacy - GUID Scheme - See Note1 below 
 - SandyBridge 
     E6420.zip 
------------------------ 
  
but still it doesn't work, when I boot from my USB installer I don't se anything connected to the SATA port of my computer 
(and yes, I did a BIOS default + data as AHCI step). 
Vincent. 
  
::edit:: exact spec is : 
Latitude E6520 with 
BIOS A19 
i7 2620 @2.7GHz 
Intel HD Graphics 
8 GB RAM 
256 GB SSD