all my post is refered to the MAC PRO EARLY 2008 3,1 DUAL XEON .... i have a friend that has a mac pro 1,1 working grate today... the bad thing in 1,1 and 1,2 mac pro they claim to be 64 bits but is false their EFI is 32bits... Don let you install yosemite or el capitan.... They use diferent motherboard and PSU.... slow buss and memory ... Mac Pro 2009, 2010 working grate no failures they are other design...
All my test i maded them in osx 10.8 , Mavericks, Yosemite and El capitan in 4 difernt SSD ... i also try with diferent riser cards from ebay , booting with only riser A exchange raisers card.... i also delided de chip bridge on mother board to replace thermal compund x5400 chipset , and also used rework staton hot air in x5400 chipset.... if you inspect the motherboard with gain lens or camera x10 you will see a lot o corrosion in diferents parts of the motherboard i think is design problem if you google you will notice that early 2008 mac pro 3,1 is the only and masive reporting of this failure
All other mac pro models solved the issue simply replace the PSU , PSU are not eternal nothing is eternal but at least we spect 3 to 4 years for a Desktop to work and more if we spend $3000 in a server grade machine....