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Probably Not for Early 2009 model Mac Pro 4.1
on March 22, 2014
This product is identical to those one can buy generically for Win PC's, only with instructions for installing in Mac Pro (a) 2008 and earlier, or (b) 2009+. As noted by several others, the instructions are very poor. The photos were more helpful. Using them, I was able to determine the motherboard on my Early 2009 model Mac Pro does NOT match the Newer instruction photos for either old or new versions of the generation 1 Mac Pro. I have the single CPU model. I spent an additional hour Googling various videos and pdf's on the topic. Most are equally glib and breezy, but some warn this is not for the faint of heart.
I used to build my own Win PCs, but I'm NOT willing to completely disembowel my Mac Pro just on the off chance there are really 2 spare Sata ports hidden somewhere else on the Mobo. If there are, they are NOT located in the upper left corner where the photos say they are. I suspect all photos were taken on older motherboards.
My solution was to order the $262 Sonnet Technologies PCIE card, from Amazon. It provides 4 external eSata ports in one slot.