Chosen Solution
Insert the original system installation DVD. Boot up holding down the “C” key. Go to the second page pull down menu to Utilities> Disk Utility> Report the internal drive setting it up as a GUID, Mac OS X Extended. Pre-Installed MacOS: X 10.5.1 (9B2117) Maximum MacOS: X 10.11.x* http://www.everymac.com/systems/apple/ma…
Vince, I also had a similar problem and struggled to use USB with El Capitan Instal for so long till I read that my version of the mac pro is not capable of running that OS. Its maximum OS is 10. 7.5. So trying this OS with either DVD or USB. You can download the ‘Lion DiskMaker’ app from the net to create a bootable USB for the installation. I think I came across an article which seems to suggest that Sierra can be installed on mac pro 2008 but couldn’t read all before I lost it. More research is needed.
