Why won't my Mac Pro 5,1 boot Snow Leopard?

Question

My old Mac Pro 1,1 died.    I replaced it with a new Mac Pro 5,1.  (dual Westmere quad core)  Apple told me it would run Snow Leopard.  The new machine recognizes my old Snow Leopard hard drives as start-up disks but does not boot past the gray screen apple logo.   I was very good about keeping the operating systems up to date.   I am pretty sure that I have OS 10.6.8 installed...   I have two Snow Leopard disks that won't boot and a Leopard disk that also shows up as a start-up disk that does not boot either.  All stall at the gray screen Apple logo. 
What gives????
I am pretty disappointed.   I need Rosetta back!
Mike

Answer 1

Mac Pro
Date introduced
Original Mac OS X included
(see Tips 1 and 3)
Later Mac OS X included
(see Tip 1)
Mac OS X Build(s)
(see Tip 2)
Mac Pro (Late 2013)
Dec 2013
10.9
13A4023
Mac Pro (Mid 2012)
Jun 2012
10.7.3
10.8, 10.8.3
11D2001, 12A269, 12D78
Mac Pro (Mid 2010)
Aug 2010
10.6.4
10.7, 10.7.2, 10.7.3
10F2521, 10F2554, 11A511a, 11C74, 11D2001
Mac Pro with Mac OS X Server (Mid 2010)
Aug 2010
10.6.4
10.7, 10.7.2, 10.7.3 (Server)
10F2522, 11A511a, 11C74, 11D2001 (Server)
Mac Pro (Early 2009)
Mar 2009
10.5.6
10.6
9G3553, 10A432
Mac Pro (Early 2008)
Jan 2008
10.5.1
10.5.2, 10.5.4
9B2117, 9C2031, 9E25
Mac Pro
Aug 2006
10.4.7
10.4.8, 10.4.9, 10.4.10, 10.5
8K1079, 8N1430, 8N1250, 8K1124, 8P4037, 8R3032, 8R3041, 9A581, 9A3129
from:
MacOSX: versions (builds) for computers
What this shows is that if you can build a verion of 10.6.8 using another Mac, starting from a "Full Retail" DVD (so that "drivers for every appropriate Mac" are included) it will boot the Mac Pro 5,1 from 2010. Users here have determined that, since the 2012 model uses the same firmware with no significant new hardware changes, it will actually boot that one as well.
Starting with a DVD that "shipped in  the box" with a different model Mac will not include the right drivers, even after update.